Question

A=1/2h(a+b) solve for a

Answer 1

So, we have:
\[
\begin{align}
A&=\frac{1}{2}h(a+b)\implies\\
2A&=h(a+b)\implies\\
\frac{2A}{h}&=a+b\implies\\
\frac{2A}{h}-b&=a
\end{align}
\]