Let the length of the side of the square be S.
Now the perimeter is given by 4S and the area if given by S^2.
We cannot say that area is 60 more than the perimeter as they have different units. We can say the same only about their magnitude.
Doing that S^2 = 4S + 60
=> S^2 - 4S - 60 = 0
=> S^2 - 10S + 6S - 60 = 0
=> S(S - 10) + 6( S - 10) = 0
=> (S + 6)(S - 10) = 0
So we have S = -6 or 10.
Now length cannot be negative, so we ignore S = -6.
The length of the side of the square is 10.
We'll note as x the side of the square.
We'll write the formula for the area of the square:
A = x^2
We'll write the formula for the perimeter of the square:
P = 4x
Now, we'll write mathematically the condition imposed by enunciation:
x^2 - 60 = 4x (area is 60 less than the perimeter)
We'll subtract both sides 4x:
x^2 - 4x - 60 = 4x - 4x
We'll eliminate like terms:
x^2 - 4x - 60 = 0
We'll apply the quadratic formula:
x1 = [4+sqrt(16 + 240)]/2
x1 = (4+16)/2
x1 = 10
x2 = (4-16)/2
x2 = -6
Since the length of the side of the square cannot be negative, we'll reject the second root x2 = -6.
The length of the side of the square is x = 10.