Greetings from Brasil...
As we have a line, the function will be given by the expression:
F(X) = AX + B
where
A = ΔY/ΔX
B = where the line intersects the Y axis
Looking at the graph we have already concluded that
B = - 3
A = ΔY/ΔX
A = (5 - 3)/(4 - 3) see attached picture
A = 2
So,
F(X) = AX + B
F(X) = 2X - 3
