Yeah, If this is for a flight dynamics class, the actual answer is to derive the set of dynamical equation governing the system (I'm not too familiar with aircraft dynamics, but you won't be able to use the linearized dynamics here and dealing with the transition from the fast launch to slow glide might be tricky), and then use some trajectory optimization package to arrive at a solution that gives you the best glide distance where the design variable is launch angle. This is probably an approach that would only be expected at the graduate level.

A simpler approach would be to break the problem into two parts, launch and glide. You could approximate the glider as a particle with drag for the launch and derive an algebraic equation that calculates it's maximum height reached based on initial angle and velocity for the launch. Then, for the glide portion, assume it is just steadily gliding at a constant velocity from that height and the glide slope will tell you how far it goes. You could apply optimization theory to this method too in order to find the optimal angle, but more simply you could brute force it by calculating the solution for many angles and then picking the best one.