Answer
$a=\dfrac{2A-bh}{h}$
Work Step by Step
Using cross-multiplication and the properties of equality, in terms of $
a
,$ the given equation, $
A=\dfrac{h(a+b)}{2}
$ is equivalent to
\begin{array}{l}\require{cancel}
2A=h(a+b)
\\\\
2A=ah+bh
\\\\
2A-bh=ah
\\\\
\dfrac{2A-bh}{h}=a
\\\\
a=\dfrac{2A-bh}{h}
.\end{array}
