A phone company has a monthly cellular plan where a customer pays a flat monthly fee of $10 and then a certain amount of money per megabyte of data used on the phone. if a customer uses 20 mb, the monthly cost will $11.20. if the customer uses 130 mb, the monthly cost will be $17.80. a)find a linear equation for the monthly cost of the data plan as a function of x, the number mb used.