sign table

I would like to see a proper sign table or similar discussion. It boils down to a single polynomial (x^2-4)^3, so I would rather talk about how to get the sign from that, instead of "brainlessly" plugging in points. We also have to explicitely repeat that -2, 2 are the boundary points.

I don't know if this is such a good thing but I'll include it as well.