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