In Problems 1-4, find dy/dt using the given values.
1. y = x3 - 3x for x = 2, dx/dt = 4
2. y = 3x3 + 5x2 - x for x = 4, dx/dt = 3
3. xy = 4 for x = 8, dx/dt = - 2
4. xy = x + 3 for x = 3, dx/dt = - 1