Testing how MathJax was hooked up
\begin{align*}
u(x,t)= &\underbracket{\frac{1}{2}\bigl[ g(x+ct)+g(x-ct)\bigr]+\frac{1}{2c}\int_{x-ct}^{x+ct} h(y)\,dy}_{=u_2}+\\[3pt]
&\underbracket{\frac{1}{2c}
\iint_{\Delta (x,t)} f(x',t' )\,dx\,d t' }_{=u_1}.
\label{eq-4}
\end{align*}