[tex]\textit{area of a trapezoid}\\\\ A=\cfrac{h(a+b)}{2}~~ \begin{cases} h=height\\ a,b=\stackrel{parallel~sides}{bases}\\[-0.5em] \hrulefill\\ h=3\\ A=18 \end{cases}\implies \begin{array}{llll} 18=\cfrac{3(a+b)}{2}\implies 36=3(a+b)\\\\ \cfrac{36}{3}=a+b\implies 12=a+b \end{array}[/tex]
