First make the problem a lot less ominous: Rotate everything 30 degrees. The mind works so much better with a level plane.
Calculate the new 'downward' force of gravity
Calculate the upward launch speed (with 15 degree launch angle, of course)
Get time via accel, speed change
Calculate sideways force of gravity
Calculate sideways launch speed
Get distance via time, accel, speed
QED, and I didn't use any 'weird' formulas except for a little rotation