Question: how do you find the slope of the line that passes through two points?
Answer: Step One: Identify two points on the line. In this example we are given two points, (15, 8) and (10, 7), on a straight line.
Step Two: Select one to be (x1, y1) and the other to be (x2, y2). It doesn't matter which we choose, so let's take (15, 8) to be (x2, y2). ... 
Step Three: Use the equation to calculate slope.