Please help me explain this question and its solution. Thank you so much!

31. A solid cylinder rolls up an incline at an angle of 20 . If it starts at the bottom with a speed of 10 m/s, how far up the incline does it travel? Solution Mechanical energy at the bottom equals mechanical energy at the top; 2 my2 -mr =mgh = h = 2 2 2 g (100.0) =7.7 m, 7.7 9.8 =22.5 m so the distance up the incline is sin 20