College Algebra, word problem...help please?

A student's grade in a course is the average of 4 test grades and a final exam that is worth three times as much as each test. Suppose a student has test grades of 91, 89, 72, and 94. Write an equation to model this situation where xis the student's grade on the final exam and yis the student's average for the course.

Then find the score they will need to receive on their final exam if they want to have a grade of 88 for the course.