I have lines such as that first line has 2 point (1,0,-1) and (4,1,-3) and when i check the sum which is (5,1,-4) it is not on the line.

Then I have line that has (2,-1,3) and (10,-5,15) and its sum is (12,-5,15) is on the line. I was wondering why the sum in some cases is on the line and in some it is not.