Write a system of equations to describe the situation below, solve using any method, and fill in the blanks.
Brian and Derek have summer jobs selling newspaper subscriptions door-to-door, but their compensation plans are different. Brian earns a base wage of $13 per hour, as well as $1 for every subscription that he sells. Derek gets $4 per subscription sold, in addition to a base wage of $7 per hour. If they each sell a certain number of subscriptions in an hour, they will end up earning the same amount. How much would each one earn? How many subscriptions would that be?
Brian and Derek will each earn $___ if they sell___subscriptions in an hour.