5m-3-2m
5 mats -3 -2mats
= 5mats-2mats - 3 (not mats)
=3mats - 3
=3m-3
When you are adding and subtracting algebraic terms it can help if you think of the letters (or groups of letters) as things 
You can only add terms (things) together is they are like terms ( the same things)