Answer :
[tex]A=\frac{f+g}{2}*h \\ \\ 2A=h(f+g) \\ \\ f+g=\frac{2A}{h} \\ \\ \boxed{g=\frac{2A}{h}-f}[/tex]
[tex]A=(f+\frac{g}{2})h\ \ \ |Multiply\ h\ by\ each\ factor\ in\ bracket\\\\
A=fh+\frac{gh}{2}\ \ |*2\\\\
2a=2fh+gh\ \ \ | Subtract \ 2fh\\\\
2a-2fh=gh\ \ \ |Divide\ by\ h\\\\
\frac{2a-2fh}{h}=g\\\\
\boxed{g=\frac{2(a-fh)}{h}} [/tex]