Physics Projectile Motion Problem (Unknown Angle)?

A cannon is fired at a mountain to start an avalanche. The muzzle speed is 1000 m/s, and the spot they're aiming for is 2000 m from the cannon, and 800 m above it, at what angle must they fire the cannon? I've wasted over an hour on this problem and it's supposed to be one of the easy ones, I've tried everything I can think of and thrown every trig identity at every equation and I can't find the angle. PLEASE HELP! Thanks :)