Question 831223
We set out to solve the problem by denoting the amount by which we increase each dimension as x.  So sequentially:

1. (18 + x)(20 + x)(6 + x) = 5280

2.  x^3 + 44x^2 + 588x - 3120 = 0

4.  (x - 4)(x^2 + 48x + 780) = 0

5.  x = 4 is clearly a solution;  the other solutions are not real numbers