Determine the number of zeros of the polynomial function.
1. f (x) = x3 + 2x2 + 1
2. f (x) = x4 − 3x
3. g (x) = x4 − x5
4. f (x) = x3 − x6
5. f (x) = (x + 5)2
6. h (t) = (t − 1)2 − (t + 1)2