See the diagram attached,
In the right-angled Triangle,
z = 250
Now, differentiate eqn(1), w.r.t. "t", (time),
also, boat is moving away from base of cliff at the rate 3 m/s, so,
also, because the height of cliff is fixed, it does not change.
Put these values in eqn(2),
so, the distance between the people (at the top of cliff) and the boat is increasing at the rate of m/s.