Building A and Building B are 30m away from each other, on the 4th floor; 35m high, someone has set up a slingshot from the window on the 4th floor of building A and is trying to sling a balloon into the window directly across from them in building B. the balloon weights 1 kg, the slingshot is 5m and there is a kinetic energy of 100 N/m.what is the angle the slingshot has to be at to make the water balloon into the window of building B?

Question

Building A and Building B are 30m away from each other, on the 4th floor; 35m high, someone has set up a slingshot from the window on the 4th floor of building A and is trying to sling a balloon into the window directly across from them in building B. the balloon weights 1 kg, the slingshot is 5m and there is a kinetic energy of 100 N/m.what is the angle the slingshot has to be at to make the water balloon into the window of building B? 
Given information
Initial Velocity= 0
Final velocity= ?
Length of slingshot= 5m
Angle=?
height of both buildings=35m
Distance between buildings=30m

I'm aware that it is a projectile motion problem, but i don't know how to go about solving it