That wouldn't mess up the acceleration on the y-axis? I was thinking that if I rotated my coordinate system the acceleration due to gravity would change direction in some way.
A projectile is fired up an incline (incline angle φ) with an initial speed vi at an angle θi with respect to the horizontal (θi > φ). (a.) Show that the projectile travels a distance d up the incline, where
d = 2*vi^2*cosθi*sin(θi-φ) / g*cos(φ)^2
(b.) For what value of θi is d a maximum...