let x = kite's horizontal distance
y = kite's vertical distance
z = kite's straight line distance
dx/dt = 20 ft/s
since y = a constant 150 ft, dy/dt = 0
the question is asking for dz/dt when z = 250 ft.
set up the relationship between the variables using Pythagoras, take the derivative w/r to time, then evaluate dz/dt.