Math Is Fun Forum

I have the equation y = x^3 + 12x - 9

Doing x = k and x = k+h I am doing differentiation

Anyway you get:

k^3 + 3k^2h + 3kh^2 + h^3 + 12k + 12h - 9 - (k^3+12k-9)
over k+h-k

Anyway I cancel the k's at the bottom and then divide the top by h

Now the answer states

3k^2h + h^3 +12h
Over h

I dont get how they got rid of the 3kh^2