A phone company has a monthly cellular data plan where a customer pays a flat monthly fee and then a certain amount of money per megabyte (MB) of data used on the phone. If a customer uses $20 \mathrm{MB},$ the monthly cost will be $\$ 11.20 .$ If the customer uses $130 \mathrm{MB}$, 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 of MB used.
b. Interpret the slope and vertical intercept of the equation.
c. Use your equation to find the total monthly cost if $250 \mathrm{MB}$ are used.