Answer
$$ b=\frac{2A}{h } .$$
Work Step by Step
To solve the given equation for $b$, we multiply both sides by $2$ and then divide both sides by $h$ as follows:
$$ A=\frac{1}{2}bh\Longrightarrow 2 A=h b\Longrightarrow \frac{2A}{h}=\frac{hb}{h }\\ \Longrightarrow b=\frac{2A}{h } .$$
