Answer
$A=4\pi r^{2}$
Work Step by Step
$r=\sqrt{\dfrac{A}{4\pi}}$ $;$ Solve for $A$
$r=\sqrt{\dfrac{A}{4\pi}}$
Square both sides of the equation:
$r^{2}=\Big(\sqrt{\dfrac{A}{4\pi}}\Big)^{2}$
$r^{2}=\dfrac{A}{4\pi}$
Take $4\pi$ to multiply the left side:
$4\pi r^{2}=A$
Rearrange:
$A=4\pi r^{2}$