Since there are X times 2 oxygen on one side and Y times 3 on the other and X and Y are both the smallest possible numbers above 0 it is quite simple to deduce that X=3 and Y=2 .
( 2X=3Y )
After writing that down the number of Al isnt equal on both sides so you muliply that getting 4 Al on the right side. My advice if you have problems with that is to raise every number until the total numbers of atoms of all types are equal on both sides by multyplying them by the number on the other side and after you finish doing that just find the biggest number you can divide those with.