Phone-Phriends charges $37 for a text-message plan with 400 text messages included. If the customer goes over the 400 messages, the cost is $0.12 per text message. They have an unlimited plan which costs $49 per month.
a. If x represents the number of text messages, and c(x) represents the cost of the messages, express c(x) as a piecewise function in terms of x.
b. Graph the function from part a.