OK, now that your homework problem is well over, here is my solution. Granted, it's a bit fancier than a typical beginner would do, but I'm an overachiever.
It computes just about everything that you could possibly want to know about the trajectory for a single angle. Then it computes and plots trajectories for several angles. You can delete anything that you don't need to know to make it simpler. The code is extremely well commented so you should have no trouble following it.
The projectile.m file is attached below these two images that it creates. If you like it, please "Vote" for my answer.