Answer
$a=\dfrac{2A}{h}-b$
Work Step by Step
Multiply 2 to both sides:
$2\cdot A=2 \cdot \frac{1}{2}h(a+b)
\\2A = h(a+b)$
Divide by $h$ on both sides:
$\dfrac{2A}{h} = \dfrac{h(a+b)}{h}
\\\dfrac{2A}{h} = a+b$
Subtract $b$ on both sides:
$\dfrac{2A}{h} - b= a
\\a=\dfrac{2A}{h}-b$
