Hi ;-)
[tex]A=\frac{1}{2}h(b+c) \ \ /\cdot2\\\\h(b+c)=2A \ \ /:h; \ h\neq0\\\\b+c=\frac{2A}{h} \ \ /-c\\\\\boxed{b=\frac{2A}{h}-c}[/tex]
Answer:
[tex]a = \frac{1}{2} h(b + c) \\ \frac{2a}{h} = b + c \\ \frac{2a}{h} - c = b \\ thank \: you[/tex]
