Okay, so here we have:
Mg + Zn
2+ Mg
2+ + Zn
From this we know: Mg is more easily oxidised than Zn
Then:
Sr + Mg
2+ Sr
2+ + Mg
From this we know: Sr is more easily oxidised than Mg.
So far we have from easiest to hardest we have: Sr - Mg - Zn
Then:
2Al + 3Zn
2+ 2Al
3+ + Zn
We know: Al is more easily oxidised than Zn, so it must be below zinc.
Lastly:
3Mg + 2Al
3+ 3Mg
2+ + 2Al
From this we get: Mg is more easily oxidised than Al, so it must be below Al.
Therefore the final order goes: Sr - Mg - Al - Zn
Understand it now?