[tex]\bf \textit{area of a trapezoid}\\\\
A=\cfrac{h(a+b)}{2}\quad
\begin{cases}
a,b=\stackrel{bases}{\textit{parallel sides}}\\
h=height\\
---------\\
A=864\\
h=24\\
a=30
\end{cases}\implies 864=\cfrac{24(30+b)}{2}
\\\\\\
864=12(30+b)\implies \cfrac{864}{12}=30+b\implies 72=30+b
\\\\\\
42=b[/tex]
