You’re going it plug in the inputs for every input for the letter x. So the first one, fx= -4(-2)+1= 9. The second one fx= -4(0)+1=1. The third one fx= -4(5)+1=19.
The area of the square:

where "a" is a length of side.
Therefore:
8=4/7(7) + b
b=4
Y= 4/7 x + 4
[You might wanna check your answer, I might be wrong. Sorry if I’m wrong.]