Question

A box with a square base and no top is to be made from a square piece of carboard by cutting 7 in. squares from each corner and folding up the sides. The box is to hold 252 in3. How big a piece of cardboard is needed?

Answer 1

so far I have the height as 7 and I'm stuck at a particular part of the problem

Answer 2

(x-7*2)^2
(x-14)^2
7(x-14)^2=252
7(x^2-28x+196)=252
7x^2-196x-196-252=0
7x^2-196x-448=0
7(x^2-28x-64)
(x^2-28x-64)

and that's where I'm stuck. I don't know how to simplify the last line