[Tex/LaTex] Breaking up an equation in the align* environment

Question

I'm trying to produce the following formatting without use of all the \quad commands to get the last equation to align nicely across the break up (a(x), b(x), and c(x) are actually really long expressions). I searched the internet for quite bit to no avail. Any ideas?

\begin{align*}
f & = \int a(x) ) dx\\
%
& = \int b(x) \\
& \quad \quad \quad \quad \cdot c(x) dx\\
%
\end{align*}

Answer 1

Use \hphantom to obtain the correct horizontal spacing without typesetting anything:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}

\begin{align*}
f & = \int a(x) dx \\
& = \int b(x) \\
& \hphantom{{}=\int b(x)}\cdot c(x) dx
\end{align*}

\end{document}