To find the balanced reaction, you have to predict the products and write the equation first.
This will be a single replacement reaction because aluminum is an element and copper nitrate is a compound.
The reaction will be:
Al + Cu(NO3)2 -> Cu + Al(NO3)3
Now, we can balance it:
2 Al + 3 Cu(NO3)2 -> 3 Cu + 2 Al(NO3)3
