how to find an intersection in 3d mode?

Hello all,

  I am trying to create that triangle shape on the attached image(proe-final),. so I drew a center line and offset it to 5units to the left and 5 ubites to the right and extended them to cross the cylindrical shape ( shown by red arrows in the image) and now I am trying to create a line in  25unit lenght from the intersect points ( the blue cross marks) so i can create that triangle but I cant get the intersect point to snap.

I would appreciate your help .

also if there is a better and easier way to create that triangle shape I would like to learn from you.

Thanks

Alex