The given equation is
[tex]A=\frac{1}{2}(B+b)h[/tex]First, we multiply each side by 2.
[tex]\begin{gathered} 2A=2\cdot\frac{1}{2}(B+b)h \\ 2A=(B+b)h \end{gathered}[/tex]Then, we divide the equation by B+b
[tex]\begin{gathered} \frac{2A}{B+b}=\frac{(B+b)h}{(B+b)} \\ h=\frac{2A}{B+b} \end{gathered}[/tex]