Finding the Point of Inflection

Hi

I need help on the following question:

1) Find the exact values of the x-coordinates of the points of inflection on the graph of $\displaystyle y=2x^4+4x^3-x^2+7x-1$

Ok this is what i have done:

$\displaystyle \frac{dy}{dx} = 8x^3 + 12x^2 - 2x +7$

$\displaystyle \frac{d^2y}{dx^2} = 24x^2+24x-2$

make $\displaystyle \frac{d^2y}{dx^2} = 0$

Now i tried using the quadratic equation but the answer i got does not look correct.

Would someone tell me what should i do instead?

P.S