We are doing derivatives of Trigonometric functions and I am STuck

Question

A surveyor with a theodolite at the top of a cliff 120m above the water observes that a boat moving directly away from him is at an angle of depression of 35 degrees and that the angle of depression is increasing at 20 degrees/minutes. Find the speed of the boat in km/h.

Answer 1

Make the drawing of this situation, and make the appropriate equations relating to its geometry. 

A trig function will be involved. Do implicit differentiation with respect to the time variable, t. REMEMBER that the argument (θ) must be in radians, so you will have to express dθ/dt in said units.