Answer :
[tex]A=\frac{1}{2}h(b+c)\ \ \ \ \ |multiply\ by\ 2\\\\
2a=h(b+c)\ \ \ \ |divide\ by\ h\\\\
\frac{2a}{h}=b+c\ \ \ \ \ |subtract\ c\\\\
\frac{2a}{h}-c=b\\\\Solution\ is\ b=\frac{2a}{h}-c[/tex]
[tex]A= \frac{1}{2}h(b+c) \\
2A=h(b+c)\\
2A=hb+hc\\
hb=2A-hc\\
b=\frac{2A-hc}{h}
[/tex]