50 is the flat fee that you pay for every time Easy-Clean cleans your house. Then they also charge you 12 dollars on top of that for the amount of rooms that they are cleaning.
So for example if you have 4 rooms in your house your equation would be
50+12(4)
12 times 4 is 48 so you would add 48 and 50 for to see how much you pay the cleaning company.