Rearrange the formula a = \pi r ^ ( 2) for r

Consider re-writing this formula as "A = pi*r^2."  The backslash symbol "\" is unnecessary.

If A = pi*r^2, and we wish to solve for r, then divide both sides by pi:

A/pi = r^2

Taking the square root of both sides,   sqrt(A/pi) = r (r must be positive in this situation).

The answer is the third choice