4bags of cement do 18m2 how many m2 can be done with 7 bags

We will represent the word bags with the letter b and the unknown with x

4b = 18m²
7b =   xm²

To work this out, we will first have to work out how many m² one bag will cover.
We know that 4b = 18m²
To get from 4b to b, we must divide by 4
Therefore to get from 18 to the missing value, we must divide by 4
18/4 = 4.5m²

Now we must work out how to get from 1 bag to 7 bags
We multiply by 7
Therefore we do the same to 4.5m²
4.5 x 7 = 31.5m²