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