You first need to compute t1 (=time to reach the lip of the cliff) and t2 (=time to reach max. elevation). The required answer is 2 (x_component of velocity) (t2-t1), since the time to go from 'touching' the lip of the cliff to max.height is the same as the time to fall back from max.height to the ground at cliff level.
You can get t1 as 60/ (x_component of muzzle velocity). You can get t2 by solving v = u + at or 0 = (y_component of muzzle velocity) - g.t . See if this works.