2.31 deduce the position of a metal within the reactivity series using displacement reactions between metals and their oxides, and between metals and their salts in aqueous solutions

Any metal higher in the reactivity series will displace one lower down from its oxide . For example...

to find out whether magnesium or copper is more reactive (therefore, higher in the series) just add magnesium to copper(II) oxide. The magnesium will displace the copper, meaning the magnesium is more reactive than the copper. Alternatively, you could add copper to magnesium oxide, No reaction would take place. This is because the copper is less reactive that magnesium (therefore, lower down in the series) as it will not displace the magnesium.

This must mean magnesium is above copper in the reactivity series.

It's the same thing with metals and a solution of their salt (the more reactive metal will displace a less reactive metal). For example...

To find out whether zinc or copper is more reactive, add zinc to a solution of copper (II) sulphate. The zinc will displace the copper (as it is more reactive), meaning it is positioned higher than copper in the reactivity series.

NOTE: in this particular reaction, the blue colour of the copper (II) sulphate solution fades as colourless zinc sulphate solution is formed.