The phone company splint has a monthly cellular plan where a customer pays a flat monthly fee and then a certain amount of money per minute used on the phone. if a customer uses 390 minutes, the monthly cost will be $169.5. if the customer uses 920 minutes, the monthly cost will be $355. a) find an equation in the form y=mx+b, where x is the number of monthly minutes used and y is the total monthly of the splint plan.