You would use the method called FOIL.
First
Outer
Inner
Last
The first are x and x, so you would do x times x which is x^2. Next are the outers, which are -3 and x, so you multiply those together to get -3x. The inbred are 3 and x, multiply those together to get 3x. The lasts are 3 and -3, multiply those together to get -9. Now your equation is x^2 -3x +3x -9, simplify this to get x^2 -9
