I will assume we have a rectangle here, since you did not specify.
In that case, we have:
SP = RQ
RS = PQ
SP = (RS)^2
Let SP = x. Thus, RQ = x, RS = x^2 and SP = x^2.
The perimeter of the rectangle is 84, so we have:
x + x + x^2 + x^2 = 84
2x^2 + 2x - 84 = 0
x^2 + x - 42 = 0
(x + 7)(x - 6) = 0
x = -7 or x = 6
We reject x = -7, since hte length cannot possibly be negative.
Thus, x = 6.
Hence, the length for each side is:
SP = 6^2 = 36
RS = 6^2 = 36
RQ = x = 6
SP = x = 6