The last term is 63 so 3 might be a zero of the function;-
f(3) = 27 - 7*9 - 3*9 +63 = 27 - 63 - 27 + 63 = 0 so 3 is a zero and
therefore (x - 3) is a factor of the volume function.
Long division of the expression by x - 3 gives the quotient:-
x^2 + 4x - 21
Factoring this:-
= ( x + 7)(x - 3)
So the dimensions of the box are, x-3, x-3 and x+7 answer.
