Is this for AS level? It looks like it might be.
1) You already have the volume. 80cm^3 is a volume.
2) Balanced equation:
2Mg + O2 ----> 2MgO
To get that you needed to write MgO first on the right hand side. You already know you need Mg on the left so write that too. Then the only element you're missing on the left that is on the right is O, which is always in diatomic form for these type of questions, so it's got to be O2.
Now you have more O on the left than you do on the right, so stick a 2 in front of the MgO so there are two oxygens on each side. Finally stick another 2 in front of the Mg on the left hand side as you now have 2 Mg on the right.
Does that help a bit?
*EDIT* Just occurred to me that this is probably a little basic for AS, apologies