sketch a picture ...

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.