The answer i got when i worked it out with my math teacher was C
Answer:
The area of the section for the picnic tables is 
Step-by-step explanation:
we know that
The area of the schoolyard is equal to the area of a rectangle plus the area of a triangle
so

The area of the triangle is equal to

F(b)=7b+8
you just need to change the x variable. make y into f(x) or in this case f(b)
C I think I hope this is correct