You can create a "ghost projectile" which basically calculates what your projectile will do and just let the physics engine do the calculations for you. You also get the added benefit of graphical feedback of the calculation.
Alternatively you can use the projectile motion formula. Just do a google search on that, there is a lot of information about it.
Since you also need to calculate where it would hit a wall, the first method is probably the easiest, since you're gonna have to trace the projectile's path and test for collision each step.
To make the calculation instantaneous, you can also scale the velocity and gravity of the "ghost projectile"