[tex]\bf \textit{area of a trapezoid}\\\\ A=\cfrac{h(a+b)}{2}~~ \begin{cases} a,b=\stackrel{bases}{parallel~sides}\\ h=height\\[-0.5em] \hrulefill\\ a=3\\ b = 9\\ A=84 \end{cases}\implies 84=\cfrac{h(3+9)}{2}\implies 84=\cfrac{h(12)}{2} \\\\\\ 84=6h\implies \cfrac{84}{6}=h\implies 14=h[/tex]
