Ineed to know the answer for 19,20,21

In the coordinate plan (-6,9) b (3,9) c (3,3) def is shown in the coordinate plan below

Dylan and dusty plan to take weekly surfing lessons together. if the 2-hour lessons are $20 per person and they plan to spend $100 each on new surfboards, what is the maximum number of lessons the two can take if the total amount spent for lessons and surfboards is at most $480?

Solve the system of equations y= 3/2 - 1, -x+ y =-3