The first step of factoring is to try to factor out a common factor.
The terms x^2 and -9x have the factor x in common.
Factor out x from both terms.
x^2 - 9x = 0
x(x - 9) = 0
Now you have a product of fully factored terms equaling zero, so you can apply the zero product property to solve.
x = 0 or x - 9 = 0
x = 0 or x = 9
Answer: x = 0 or x = 9