Aaliyah owns a small textile company that produces shirts. On a particular day, Aaliyah has one worker come into the shop to make shirts. She must pay the worker $28 per hour and also pay $2 per shirt for material costs. The worker created an average of 4 shirts per hour and Aaliyah's total expenses for labor and materials were $144. Write a system of equations that could be used to determine the number of hours the worker worked and the number of shirts the worker made. Define the variables that you use to write the system.