3x^3 = 6x^2 - 45x
3x^3 - 6x^2 - 45x = 0 you need to set it = to 0
3(x^3 - 2x^2 - 15x) = 0 divide by 3
x^3 - 2x^2 - 15x = 0
I used long division and got (x^3 - 2x^2 -15x) can be divided by (x+3)
(x+3)(x^2 - 5x) = 0
x(x+3)(x - 5) = 0
x = 0, -3, 5
b. is the answer