If x^2+y^2=8100 and dy/dt=3, find dx/dt when y=72.

A man starts walking north at 5ft/s from a point P. Five minutes later, a woman starts walking south at 4ft/s from a point 1300ft due east of P. At what rate are the people moving apart 20min after the woman starts walking?

A particle moves along the curve y=(19+x^3)^(1/2). As it reaches the point (5, 12), the y-coordinate is increasing at a rate of 3cm/s. How fast is the x-coordinate of the point changing at that instant?

Thank you!