Question

factor x^2-8x=9=0

Answer 1

Assuming you meant

$$
x^2-8x-9=0
$$

What two numbers, a and b multiply to give you -9 and subtract to give you -8

$$
ab=-9\\
a+b=-8
$$

Then

$$
x^2-8x-9=(x+a)(x+b)=0
$$

Then set each term to 0, solve for x:
$$
x+a=0\\
x+b=0
$$

x=-a and x=-b are your solutions.

That's it!