Find the zeros, just equal the equation to 0.
x³ - 5x² - 9x + 45 = 0
Factor:
x².(x - 5) - 9.(x - 5) = 0
(x² - 9).(x - 5) = 0
It's a multiplication, so, if one term goes to 0, everything goes too.
x² - 9 = 0
x² = 9
x = + or - √9
x' = 3
x'' = -3
x - 5 = 0
x = 5
Now we have three roots: -3, 3, 5.