since this is going to be an algebraic equation we have to put a variable so ill go with the variable N. Now making an algebraic equation is pretty easy once you have the concept but they will progress over time
N is our variable aka our width
and 3 is our number aka our length
now since it, 2 less than that will be in our equation as 2n
as you know we have to use our variable and our number which is just going to subtract and our answer is 2n-3
when making equation you may use any letter that you would like it does not matter