hi, i'm stuck on a question and would like some help please:

find the values for k for which the quadratic equation $\displaystyle 3x^2 + (6 - k)x + 12 = 0$ has equal roots. i used the discriminant theory and got it to $\displaystyle \sqrt{36 - 12k + k^2 - 144}$ = $\displaystyle \sqrt{-108 - 12k + k^2}$ and then $\displaystyle - 12k + k^2 = 108$ but i'm really not sure if any of this is right. could someone show me if this is the right way?

thanks, Mark