Check the picture below.
now, looking at the picture's scaling, from the lower-left-corner up to 20 is 5 squares, the rectangles you see in the picture are 40 units each, namely 10 squares. Every square is 4 units, it goes up 12 squares, namely 48 units, and goes to the right by 30 squares plus 4 more, 34 squares, that's 136 units.
well, if we were to make a triangle for the slope, check the picture, it'd look like that.
Why is the slope negative? well, because is going down from left to right.
