Instead of setting itemindent, you can change the values for leftmargin and labelindent:
\documentclass[letterpaper]{article}
\usepackage{fullpage}
\usepackage{amsmath,amssymb,amsthm,enumitem}
\begin{document}
To convert from general form to standard form, we will follow these steps:
\begin{enumerate}[leftmargin=*,labelindent=16pt,label=\bfseries Step \arabic*.]
\item Ensure the coefficient of the terms in $x^2$ and $y^2$ are 1 respectively.
\item Group the terms containing $x$ and $y$ respectively. Move the constant to the RHS.
\item Take half of the coefficient of the linear terms in both $x$ and $y$, square it and add it to both sides.
\item Rewrite the result in factored form.
\end{enumerate}
\end{document}
In fact, if some of your lists will have 10 or more items, the following settings would be preferable:
\documentclass[letterpaper]{article}
\usepackage{fullpage}
\usepackage{amsmath,amssymb,amsthm,enumitem}
\begin{document}
To convert from general form to standard form, we will follow these steps:
\begin{enumerate}[labelwidth=1.5cm,labelindent=10pt,leftmargin=2.2cm,label=\bfseries Step \arabic*.,align=left]
\setcounter{enumi}{8}% just for the example
\item Ensure the coefficient of the terms in $x^2$ and $y^2$ are 1 respectively.
\item Group the terms containing $x$ and $y$ respectively. Move the constant to the RHS.
\item Take half of the coefficient of the linear terms in both $x$ and $y$, square it and add it to both sides.
\item Rewrite the result in factored form.
\end{enumerate}
\end{document}
With the help of the layouts package and the following simple code
\documentclass{article}
\usepackage{layouts}
\begin{document}
\begin{figure}
\listdiagram
\caption{List parameters}
\end{figure}
\end{document}
you get a diagram showing the various lengths associated to a list:
the enumitem package adds another parameter \labelindent for the blank space from the margin of the enclosing list/text to the left edge of the label box. Here's the dependencies between the relevant horizontal parameters:
\leftmargin + \itemindent = \labelindent + \labelwidth + \labelsep
Only modifying itemindent changes the indention for the first line of the first paragraph of each \item, and this produces the undesired result in your example code; modifying leftmargin changes the indention for all the lines. Compare the results in this simple example:
\documentclass{article}
\usepackage[showframe]{geometry}
\usepackage{enumitem,lipsum}
\begin{document}
\begin{enumerate}[itemindent=3cm]
\item text
\item \lipsum[2]
\end{enumerate}
\begin{enumerate}[leftmargin=3cm]
\item text
\item \lipsum[2]
\end{enumerate}
\end{document}
Somehing like this? (two && before the \mbox)
\documentclass[letterpaper]{article}
\usepackage{fullpage}
\usepackage{amsmath,amssymb,amsthm,enumitem}
\usepackage[dvipsnames]{xcolor}
\newcommand{\red}[1]{%
{\color{OrangeRed}#1}}
\begin{document}
Let us apply completing the square to the standard form of a quadratic equation, $ax^2+bx+c=0$.
\begin{align*}
ax^2+bx+c&=0 &&\text{Divide by $a$, the leading coefficient.}&\\[1.25ex]
x^2+\frac{b}{a}x+\frac{c}{a}&=0 &&\text{Transpose the constant term to the RHS.}&\\[1.25ex]
x^2+\frac{b}{a}x&=-\frac{c}{a}&&\text{\red{Add $\left(\frac{1}{2}\left(\frac{b}{a}\right)\right)^2$ to both sides.}}&\\[1.25ex]
x^2+\frac{b}{a}x+\frac{b^2}{4a^2}&=-\frac{c}{a}+\frac{b^2}{4a^2}&&\text{Factor the LHS and simplify the RHS.}&\\[1.25ex]
\left(x+\frac{b}{a}\right)^2&=\frac{b^2-4ac}{4a^2}&&\text{Take the square root of both sides.}&\\[1.25ex]
x+\frac{b}{a}&=\pm\frac{\sqrt{b^2-4ac}}{2a}&&\text{Solve for $x$.}&\\[1.25ex]
x&=\frac{-b\pm\sqrt{b^2-4ac}}{2a}&&\text{\textbf{Quadratic Formula}.}&
\end{align*}
\end{document}
Best Answer
You can use
\hfillrather than\hspaceand it should work.