Answer
$A=4\pi{r^2}$
Work Step by Step
Square both sides to obtain:
$r^2=\dfrac{A}{4\pi}$
Multiply $4\pi$ on both sides to obtain:
$\require{cancel}
4\pi \cdot r^2 = \dfrac{A}{4\pi} \cdot 4\pi
\\4\pi{r^2} = \dfrac{A}{\cancel{4\pi}} \cdot \cancel{4\pi}
\\4\pi{r^2} = A$
