Answer
$$b = 2A - a$$
Work Step by Step
To solve this equation for $b$, we need to isolate $b$.
We first divide both sides by $\frac{1}{2}$, which means we will multiply each side by the inverse, which is $2$:
$$2A = a+b$$
We then subtract $a$ from each side:
$$b = 2A - a$$