Pls help me answer this :’)

The fence around a quadrilateral-shaped pasture is 3a^2+15a+9 long. three sides of the fence have the following lengths: 5a, 10a-2, a^2-7. what is the length of the fourth side of the fence?

What is the total area of the figure?

240 seventh graders and two-thirds of the students participate in after-school activities how many students participate in after-school activities