what you have to find is what numbers add to -6 and multiply to 9. these numbers would be -3 and -3 (-3+-3=-6, -3*-3=9). substitute these in for -6x as x^2-3x-3x+9=0. then factor out c and three to get x(x-3)-3(x-3)=0. x-3=0 in both cases, so x is three.
