Using calculus and reasoning prove how, step by step, the following is true.

a, b, and c are >= 0

If a^2 + b^2 + c^2 + abc = 4 show that ab + bc + ac - abc <= 2

This is a calc 3 problem, vectors and planes in spaces, so i'm thinking i have to find the plane for the first equation and then show relationships to the other inequality. Basically, I'm pretty much lost and have no clue where to start. Any help is appreciated. Thanks.