Introductory Algebra for College Students (7th Edition)

$$b = \frac{2A}{h} - a$$
To solve for $b$, we need to isolate $b$: We first divide each side by $\frac{1}{2}$, which means we need to multiply by the inverse of $\frac{1}{2}$, which is $2$: $$2A = h(a + b)$$ We then divide each side by $h$: $$\frac{2A}{h} = a + b$$ We now subtract $a$ from each side: $$b = \frac{2A}{h} - a$$