Camile has a plan for her cell phone in which she pays a fixed rate for calls and a certain amount for each text message sent. When she sent
300
messages, her total bill was
$
21.60
and when she sent
525
messages, her total bill was
$
26.10
.

Write a function of the total cost,
f
(
x
)
based on the number of messages sent,
x
.