Answer
$$
\frac{2A-bh}{h}, \ \ 21\bar{0}
.$$
Work Step by Step
To solve the given equation for $a$, we multiply both sides by $2$ and then we follow the following steps:
$$ A=\frac{(a+b)}{2}h \Longrightarrow 2 A=(a+b) h \Longrightarrow 2 A=ah+bh\\ \Longrightarrow 2A-bh=ah\\ \Longrightarrow a=\frac{2A-bh}{h}.$$
Now, we have
$$
a=\frac{2A-bh}{h}=\frac{2\times5502-183\times28}{28}=21\bar{0}
.$$
