perpendicular means the slope will be -1/3 (negative reciprocal of 3)
... 10 is the y-intercept of the upper line, so the perpendicular will match it
... the perpendicular intersects the top line at (0,10)
the intersection with the bottom line is when 3x=(-1/3)x+10 ... or x=3 ... so the intersection is (3,9)
the distance is between the two intersections ... d^2=(3-0)^2+(9-10)^2
