A=1/2bh (find b)

$$\\A=\frac{1}{2}bh\\\\
\frac{2A}{h}=b$$

Divide by h, then divide by 1/2 (same thing as multiplying by 2), and you're left with

$$b = \frac{2A}{h}$$

This is actually the formula for finding the area of a triangle. So A=1/2bh is actually Area-.5(base x height).