3.2 because l is 20 so you would put your constant on another side and subtract it from 20 leaving you with 8 = 2.5m and then you would just solve for m from there.
l = 20
20 = 12 + 2.5m
2.5m = 20 - 12
2.5m = 8
m = 8 ÷ 2.5
m = 3.2cm
