I think it is easy... don't get me wrong if i make a mistake. First thing first eliminate Elements/Molecules that are just there... doing nothing. In this case u have none. You have 2 know this: All elements in standard form have O.N. #=0 meaning Al(s)=0        --->   Zn(s)=0. Now the ions have that are charged will have same O.N. # as the charge on it so... Zn2+= 2+ ---> Al3+= 3+    notice the change of O.N.# as the reaction occurs.
 0           2+                     3+            0                     Determine the change of O.N # in Ratio:
Al(s)  +   Zn(aq) 2+   --->  Al(aq) 3+   Zn(s)               In this case Al: 3
                                                                                           Zn: 2     --> flip the ratio and make them the co-efficient of the elements in reaction...

2Al(s) + 3Zn(aq)2+    --->  2Al(aq) 3+  + 3Zn(s)      I did this long time ago, but I know  that most of it is correct maybe im messing up near the end. Don't count on it