You've got the right idea. You have the points:
(60, 40) and (100, 60). You want to find the slope, as you know, but you found the slope incorrectly. The slope is the difference in the y-coordinates divided by the difference in the x-coordinates:
So now you know the slope, m. Use point-slope form (with either point) to write the equation. Point-slope form:
where are the coordinates of EITHER of your two points, your choice.