A little trick I like to use sometime is cosp()
cosp(a,b,x) basically will move a platform from a to b in timeframe x and then back again , either up and down or left and right .
ex: system.everytick-->sprite.set x cosp(50,400,time/3) or sprite.set y cosp(100,400,time/3)
its fast and easy and only uses 1 event , you can change the time it takes with the last parameter .