Can anyone help me ? I got stuck Find the derivative of f(x) = -12x^2 + 9x at x = 6.

Question

twalt13 · Answer

find f(x) = -12x^2+9x
find f(x+h) = -12(x+h)^2 + 9(x+h)
                 = -12x^2-12h^2+9x+9h

twalt13 · Answer

Now i gotta put that  into this formula$$f(x+h)-f(x)\div h$$

twalt13 · Answer

so... 
-12x^2-12h^2+9x+9h -(-12x^2)+9x)
-12x^2-12h^2+9x+9h +12x^2-9x    combine like terms

-12h+9h/h     here is where i got stuck

twalt13 · Answer

*-12h^2+9h/h

hartnn · Answer

find f(x+h) = -12(x+h)^2 + 9(x+h)
                 = -12x^2-12h^2+9x+9h ********incorrect

$(a+b)^2= a^2+2ab+b^2 $ 
where did your '2ab' term go ?

twalt13 · Answer

idk i didnt know there was a 2ab term

hartnn · Answer

now you know, 
so whats (x+h)^2= ... ?

twalt13 · Answer

@hartnn i solved that

hartnn · Answer

i am glad :)