Kayla is working two summer jobs, making $10 per hour lifeguarding and $21 per hour tutoring. Last week Kayla worked 4 times as many hours tutoring as she worked lifeguarding and earned a total of $188. Write a system of equations that could be used to determine the number of hours Kayla worked lifeguarding last week and the number of hours she worked tutoring last week. Define the variables that you use to write the system.