A=4pi r^{2} solve for r

A = 4πr² Divide both sides by 4π

A / 4π = r² Find the square root of both sides

√ (A / 4π) = √ (r²) Cancel out the square with the square root

√ (A / 4π) = r Switch the sides to make it easier to read

r = √ (A / 4π)