Could you check the question please
www.jiskha.com/questions/1805638/Question-1-Find-the-perpendicular-distance-of-a-point-C-c-fron-the-straight-line#1914132
If (r-a)×b = 0 then (r-a) and b must be parallel, since the sine of the angle between them is zero.
As for the 2nd one, if you meant to say that (r-a)×d = 0 as well, then b and d are parallel. What do you know about the distance between parallel lines in 3D?