One root of the equation x^3+ax^2+7=0 is x=-2. I need to find the value of a.

I tried to do this by using b^2-4ac to find the discriminant, but this didnt work as i dont have a or b just c=7.

I also rearrnaged the equation to give a on its own as -3, but i substituted this back into the equation, but i didnt get 0.

Thanks gor your time.