# Math Help - Cubic polynomials

1. ## Cubic polynomials

Eh. Here's the problem:

"A cubic polynomial is given by $f(x)=ax^3+bx^2+30x+k$ where a, b, and k are constants. The function f(x) has a local minimum at (-1,-10) and a point of inflection at x=2.

a) Find the values of a, b, and k.

b) Use the second derivative to verify that (-1,-10) really is a local minimum for your function.

c) Find the location of any local maximums for your function. Verify that they are indeed a local maximum."

Here's what I've done so far..

I've found the first derivative, $f(x)= 3ax^2 + 2bx + 30$
and the second derivative, $f(x)= 6ax + 2b$.

I then plugged the value of x=1 into the first derivative, getting $3a-2b+30$.
I also plugged the inflection point of x=2 into the second derivative and got
$12a+2b$.

From there, I can't seem to remember what to do. Help, please? D:

2. solve the following linear equations :
$f(-1)=a(-1)^3+b(-1)^2+30(-1)+k=-10$
$f'(-1)= 3a(-1)^2 + 2b(-1) + 30=0$
$f''(2)= 12a + 2b=0$
to get the value of a, b, and k.