2. Let V be the volume of the tetrahedron with vertices a, b, c, r.
Let S be the area of the triangle with vertices a, b, c.
Then, the distance d is V=(1/3)Sd.
V and S can be computed using external products.
Draw a perpendicular line from r to the plane, meeting the plane at the point h. You can express h as h=sa+tb+uc where s+t+u=1.
Determine s, t, u by (r-h).(a-b)=0, (r-h).(b-c)=0.
The distance d is the length of the vector r-h.
3. Find 2 points on the line and Use 2.