f(x)=6x^2+9x-6

Where does this function intersect the x-axis (i.e. what are the roots or zeroes of f(x))?

Solve by factoring using the Box Method. When I use the method that is shown on this website Factoring Quadratics: The Hard Case for this particular problem I end up with this as my factored down version of the equation.

(6x-3)(3x+6)

But that is wrong because I would end up with 18x^2 instead of 6x^2.

My question is that does using the method on that website to factor down a quadratic work for this equation? If so, could someone tell me what I'm doing wrong.

Thanks