Answer
$$b= \frac{2A}{h}-a .$$
Work Step by Step
To solve the given equation for $b$, we multiply both sides by $2$ and then we follow the following steps:
$$ A=\frac{(a+b)}{2}h \Longrightarrow 2 A=(a+b) h \Longrightarrow 2 A=ah+bh\\ \Longrightarrow 2A-ah=bh\\ \Longrightarrow b=\frac{2A-ah}{h}=\frac{2A}{h}-a .$$
