needed
If the expression (x-a) is a factor of f(x), then f(a) = 0.
So just plug each of them into f(x)
f(5) = 160
f(-3) = 0
f(15) = 4320
x^3 + 5x^2 - 9x - 45
We can factor this by grouping
x^2 (x + 5) - 9 ( x + 5) [ ( x + 5) is the common factor]
(x + 5) ( x^2 - 9 )
(x + 5) ( x + 3) ( x - 3)
So....only ( x + 3) is a factor
