If the two lines intersect, then it is clear that D = 0
If they do not intersect and are parallel, then D corresponds to the distance between a point from one of the two lines and the other line and is given by the formula that find the distance from a point to a line.

Find the distance between the lines r: 4x-3y+9=0 and s: -4x+3y+1=0

D =