Question

X(x+9)(x-1)=72. find x

Answer 1

X(x+9)(x-1) = 72
lets expand the equation:
(x^2 + 9x)(x - 1) = 72
x^3 - x^2 + 9x^2 - 9x - 72 = 0
x^3 + 8x^2 - 9x - 72 = 0
and factorize again:
x^2(x + 8) - 9(x + 8) = 0
(x + 8)(x^2 - 9) = 0
that is:
(x + 8)(x + 3)(x - 3) = 0
double check that x^2 - 9 = (x + 3)(x - 3), that is a subtraction of squares easily factorizable:
So we have:
(x + 8)(x + 3)(x - 3) = 0
which solutions are:
x = -8, x = -3, x = 3