The equation given for the area, A is
A = s^2.
Then, you are asked to solve for the side, s.
You need to extract square root from both side which drives to:
√(s^2) = +/- (√A)
=> s = +/- (√A)
but given that s is a length it can only be positive, and
s = (√A)
Answer: s = √A