Question

(20-r)^(1/2)= r

Answer 1

assuming you want to solve for r, square both sides to get

20-r = r^2
r^2 + r - 20 = 0
(r+5)(r-4) = 0

and it's all downhill from there.
Just remember: √25 = 5, not -5

Answer 2

Thank you!