I have a homework question I cannot solve. The question is:

For what values does the graph of f(x) x^3 + 3x^2 + x + 3 have a horizontal tangent.

I though I could take the derivative of this function and set that to zero and solve using the quadratic formula but that does not seem to work. Does anyone have any hints to lead me to the right solution?

EDIT: Can someone move this thread to the urgent homework help? I posted this in the wrong section