Mixing X and Y (horizontal and vertical)
Since the horizontal motion of the projectile is completely independent from the vertical motion, you can actually deal with the math of the vertical separately from the math of the horizontal.
In support of all the students who are displaced from school due to the Corona virus. Access to physics zone and chemistry zone lessons are now available free of charge. This will be maintained at least through August 1st 2020. Learn and be well.
Since the horizontal motion of the projectile is completely independent from the vertical motion, you can actually deal with the math of the vertical separately from the math of the horizontal.