Size of Squares

First, get a sheet of tin and a pair of snips.....

This solution is obvious since 35 can only be formed as 7x5x1, so you must cut 1x1 squares from each corner. That said, the general way to solve this is:

Let x by the side length of the square you remove. The box volume (V) is determined as

V = (9 - 2x)(7 - 2x)x
35 = 63x - 32x^2 + 4x^3
0 = 4x^3 - 32x^2 + 63x - 35
0 = (x-1)(4x^2 -28x +35)

x = 1, (7 + sqrt(14))/2, (7-sqrt(14))/2 or 1, 5.371, 1.629
The 5.371 answer is impossible since that would be bigger than the sheet, so delete that one.

Thanks.

Thanks, but I don't understand it.