The distance from a point described by the vector s to the line
r = a + tb
is |(s−a)×b| if we make b a unit vector
or, more generally, |(s−a)×b|/|b|
This video works through an example carefully and clearly. Just one word of warning: the video uses a for the direction vector of the line (what we usually call b):
and b for what we’ve called s−a.