How to make a perpendicular to 2 lines?

I have two lines crossing at point X. I need to draw a third line starting at X which is perpendicular to both lines. A simple vector product of those, right? So what is the easiest way to do that?

Are you talking about doing this with vanilla Rhino, or scripting?

In vanilla Rhino, create a CPlane 3 point with point X as the origin and some point on each line. Then use Line>Vertical and start your line at point X…

For scripting, have a look at the rhinoscriptsyntax VectorCrossProduct method…