Inflection points
posted by Kay .
Consider the function f(x)=12x^5+30x^4–300x^3+2. f(x) has inflection points at (reading from left to right) x=D,E, and F. where D is___, E is____, and F is____.
I took the second derivative of the function and got 120x(2x^2+3x15). From that I set it to equal and tried finding x. One is obviously zero.
Then I tried using the quadratic formula to get the other two x's. I got [3 + sqrt(129)]/4 and [3  sqrt(129)]/4
However I plugged the answer into my online homework site and it says that is incorrect. Can someone help?

Assistance needed.
Please type your subject in the School Subject box. Any other words are likely to delay responses from a teacher who knows that subject well. 
I agree with you.

Writeacher, thanks for the tip...I was never really sure whether I was supposed to type my actual class subject or the subject material. If it's ok, I might submit this question again with a different school subject.