If you draw a diagram, you will see this is a very easy problem.
Let the side of the square be x feet.
Then, the total area of the square minus the area occupied by the building is 15,300 ft2.
That is: x2 - (80 . 20) = 15300
=> x2 - 1,600 = 15,300
x2 = 15,300 + 1,600
x2 = 16,900
x - √16.900 = 130 ft