18*m +fee = charge
at 2 months the charge is 81 so substitute in
18*2 +fee = 81
36+fee=81
subtract 36 from both sides
fee = 45
for 5 months what is the charge
18*m +fee = charge
18*5 +45 = charge
90 + 45 = charge
135
Answer: the initial fee is $45 and the 5 month fee is $135