You're looking for the equation of the line through the two yellow points, whose coordinates are (4, 0) and (8, 9).
Compute the slope:
(9 - 0) / (8 - 4) = 9/4
Using the point-slope formula, the equation of the line is
y - 0 = 9/4 (x - 4)
or
y = 9x/4 - 9
