# Thread: Finding functions of g(x) and f(x)

1. ## Finding functions of g(x) and f(x)

I'm really stuck on this question, any help would be appreciated.

If f(3)=-2 and f'(3)=5, find g'(3) if: g(x)=(3x^2)-5f(x)

the answer is: -7

thanks in advance

2. ## Re: Finding functions of g(x) and f(x)

Hey JellyOnion.

Hint: Try differentiating g(x) by noting that d/dx(h(x) + j(x)) = h'(x) + j'(x) and substitute in f'(3) where appropriate.

3. ## Re: Finding functions of g(x) and f(x)

Not sure i fully get what you mean chiro

4. ## Re: Finding functions of g(x) and f(x)

g(x) = 3x^2 - 5 f(x)

g'(x) = (3x^2 - 5 f(x)) '
g'(x) = (3x^2)' + (-5 f(x)) '
g'(x) = 3 * 2x + -5 * f ' (x)
g'(x) = 6x - 5 f ' (x)

Now set x = 3

g'(3) = 6*3 - 5 f ' (3)
g'(3) = 18 - 5 * 5
g'(3) = 18 - 25
g'(3) = -7

5. ## Re: Finding functions of g(x) and f(x)

Thanks mathguy25