These are a few problems from my chapter review that I was unable to solve. If you could help, I would greatly appreciate it. Thank you!

19. The volume of a square pyramid is related to the length of a side of the base s and the height h by the formula V=(1/3)(s^2)h. How is dV/dt related to ds/dt if h is constant?

20. If xy+x=12 and dx/dt =-3, then what is dy/dt when x=2 and y=5?

16. At the two points where the curve x^2-xy+y^2 = 36 crosses the x-axis, the tangents to the curve are parallel. What is the common slope of these tangents?