In going from (5, 5) to (10, 8), x (the run) increases by 5 and y (the rise) increases by 3. Thus, the slope of the line connecting the first two points is m = 3/5.
In going from (1, 13) to (4, 8), x (the run) increases by 3 and y (the rise) decreases by 5. Thus, the slope of the line connecting the first two points is m = -5/3
Because these results are negative reciprocals of one another, the two lines are PERPENDICULAR to one another.
