How to I extend a vector?

Hi, I'm relatively new to Matlab and I I have been finding hard searching around help concerning this particular problem of mine!So..Is there a way to extend a vector in Matlab to the point of a specific surface.I have the vector defined but I want to extend it all the way to xy (e.g.) surface so that I can find out the coordinates of that point that the extended vector of mine will reach the "ground". Thanks in advance!