How to make a perpendicular to 2 lines?

Hi!
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?
Thanks.

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…