The given line has a slope of -1/3 so anything parallel to that given line is also going to have an equal slope (of -1/3)
To find the slope of that line shown, notice how we drop down 1 unit and the move to the right 3 units when we go from the left point to the right point. Alternatively, you can use the slope formula m = (y2-y1)/(x2-x1) to get the same result.
