The coefficient in this problem would represent the cost of the shirt with the tax!
Think about it like this
If a shirt costs 25 dollars and the tax is .08 per dollar then you would multiply 25*.08 to get two dollars
So in addition to the 25 dollars for the shirt, you would also have to pay the two dollars extra in taxes.
Meaning in total you would pay 27 dollars
So by adding the .08 to the whole which is in this case represented by 1
You would be accounting for both the price of the shirt and the tax!