Greetings from Brasil...
According to the statement of the question, we conclude that the area of a square is requested, whose value is 171 in²
The area of a square is given by the expression:
A = S²
where S is the side of the square - just what we want to know
So:
A = S²
171 = S²
√171 = S
S ≈ 13.08in