The perimeter of an object is given by the sum of all the sides of the object. Thus the perimeter of a quadrilateral is the sum of the four sides of the quadrilateral.
Given a quadrilateral with sides: n, 2n - 12, 3n + 4, n + 7
The sum is n + 2n - 12 + 3n + 4 + n + 7 = 7n - 1 = 90
Thus, 7n = 90 + 1 = 91 and n = 91 / 7 = 13.
Therefore, the length of the side KL is n = 13 centimeters.