#### Answer

$b=\frac{2A}{h}-a$

#### Work Step by Step

Area of Trapezoid $A=\frac{1}{2}h (a+b)$
Solving for b
$A=\frac{h}{2} (a+b)$
Multiply both sides by $\frac{2}{h}$
$\frac{2}{h} \times A=\frac{2}{h} \times \frac{h}{2} (a+b)$
$\frac{2A}{h} = a+b$
Add $-a$ to both sides
$\frac{2A}{h}-a = a+b-a$
$\frac{2A}{h}-a = b$
$b=\frac{2A}{h}-a$