Answer
$a=\frac{2A}{h}-b$; area of a trapezoid formula
Work Step by Step
Multiply each side by $2$ and divide each side by $h$. Then subtract $b$ from each side.
$A=\frac{1}{2}h(a+b)$ for $a$
$\frac{2A}{h}=a+b$
$a=\frac{2A}{h}-b$
This is the formula for the area of a trapezoid.