[tex] \bf \textit{area of a trapezoid}\\\\
A=\cfrac{h(a+b)}{2}~~
\begin{cases}
a,b=\stackrel{bases}{parallel~sides}\\
h=height\\
---------\\
a=10\\
h=6\\
A=51
\end{cases}\implies 51=\cfrac{6(10+b)}{2}
\\\\\\
51=3(10+b)\implies \cfrac{51}{3}=10+b\implies 17=10+b\implies 7=b [/tex]
