If you look carefully at the points and the line, you will see that
there are two points that are exactly on the line.

They are (3600,230) and (4000,260)

We will use x for engine speed and h for horsepower since your
graph has horizontal axis as engine speed and vertical axis as
horsepower.  At the end we'll switch to the letters s and h.

So we use the slope formula and the point-slope formula.





The we substitute in the point-slope formula for the equation
of a line:







  <-- slope-intercept form y=mx+b

Finally, since y represents horsepower, we replace y by h and since
x represents engine speed, our final equation is

 

Edwin