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?