Rectinilear motion

Sara launches a model rocket in an open field near her house. Its trajectory is described by the function y=60ln(x+1)-6x for x=[0,36.15] where y is the rocket's height (in feet) above the ground and x is the horizontal distance (in feet) between the launch site and a point directly below the rocket.

Given that the rocket's horizontal velocity (dx/dt) is 50 ft/s when it's directly over a point 20 (horizontal) feet from the launch site, determine the rocket's vertical veloctiy (dy/dt) at that point.

I don't know how to go about solving this question, because normally the position is given as a function of time. Can someone help me? Thanks.

 

oh yeah!! i was overthinking it way too much. thanks though!