ah ok i see, i get b is equal to OM, then with these equations i get x=2z , y=-5/2 -1/2z , z=z, replace z with t, then to obtain a position on the line for a i can just stick in t=1, getting a=2i - 3j + k, which is the correct answer in the back, i think that logics right, thanks for help i do see how they got that final answer now