Well, let's first find our rate of bags to meters.
If 12 bags of plaster covers 54m^2, then we need to divide 54 by 12 to figure out how many meters 1 bag of plaster covers. We will then use that rate and divide 36 by it in order to solve for how many bags of plaster is required.
54 / 12 = 4.5.
This means each bag of plaster covers 4.5m^2.
Divide 36 by 4.5;
36 / 4.5 = 8.
This means that we will need 8 bags of plaster to cover 36m^2.
I hope this helps!