The line passes through points (0,.5,1) and (2,1,-3) find parametric equations and symmetric equations for the line

I went ahead and subtracted (2,1,-3) from (0,.5,1) and ended up getting right answer, later to find out i subtract other way around to get right answer. Since its matters what which one subtracts from which one how do i determine which one to subtract from, the one with a higher z value? sorry the teacher never told me