The phone company a fee 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 420 minutes, the monthly cost will be $231. if the customer uses 820 minutes, the monthly cost will be$431. find and equation in the form y=mx+ b, where x is the monthly minutes used and y is the total monthly cost of the a fee and fee plan