with slope always remember rise over run, vertically, the dots are 5 points away from each other so that is your rise. Horizontally they are only 2 away so that's your run. the answer is negative because the slope goes down to the right. If it went up to the right then the slope would be positive.
Explanation: The formula for slope is rise/run. If you start at the point (1, -2), you can go up 5 and left 2 to get to the other point. Since the line goes downwards, the slope must be a negative. This leaves you with -5/2. Hope this helps!
