Josiah works at an electronics store as a salesperson. Josiah earns a 2% commission on the total dollar amount of all phone sales he makes, and earns a 7% commission on all computer sales. Josiah had $300 more in computer sales than in phone sales and earned a total of $156 in commission. Write a system of equations that could be used to determine the dollar amount of phone sales Josiah made and the dollar amount of computer sales he made. Define the variables that you use to write the system.