#### Answer

$B=\dfrac{2A}{h}-b$

#### Work Step by Step

$A=\dfrac{1}{2}h(B+b)$ $,$ for $B$
Take the $2$ to multiply the left side:
$2A=h(B+b)$
Take $h$ to divide the left side:
$\dfrac{2A}{h}=B+b$
Take $b$ to subtract the left side:
$\dfrac{2A}{h}-b=B$
Rearrange:
$B=\dfrac{2A}{h}-b$