It doesn't matter which of your points you call point 1 and which you call point 2 as long as you remain consistent while working the problem. Let's say (15,1) is point 1, and (4,2) is point 2.
Then simplify and solve the equation for y. The slope will be the coefficient on the x term, because by solving for y you put the equation into the slope-intercept form,
I'll leave the arithmetic to you. I'll also leave it as an exercise for the student to re-identify the points so that (4,2) is point 1, etc., and then prove that the resulting standard and slope-intercept forms of the equation are the same regardless of the order of the points.