An internet service provider charges $18 per month plus an initial set up fee. one customer paid a total of $81 after 2 months of services .
(a) write an equation modeling this situation.
(b) what is the initial setup fee?
(c) how much does it cost after 5 months of service?

Ok so
cost=setupfee+costused
costused=(number of months) times (cost per month)
if we represent number of months as x
we know cost per month is 18
18x is costused
 and in 2 m onts, he paid 81 so
81=startup fee +18x
2 monts so x=2
81=startupfeee+36
subtract 36
45=startup fee

5 monts is x=5
45+18(5)=45+90=135

A. cost=18x+45
B. $45
C. $135

Whose mistake? can you give more details to what the question is asking?