Symbols – Calculating Partial Derivative with Respect to Three Variables

Question

I just started learning LaTeX and I am facing problem in writing the following equation, can anyone please help me out?

Answer 1

A simpler code with the diffcoeff package. The order of differentiation is automatically computed — I suppose k+l is k_1+k_2+l:

\documentclass{article}
\usepackage{mathtools, diffcoeff}
\DeclarePairedDelimiter\abs{\lvert}{\rvert}

\begin{document}

\[
\abs*{\diffp[k_1,k_2,l]{u_0}
           {x_1, x_2, t}} \leq C
\]

\end{document}