Think of these like coordinates, but instead think in vectors. So for A - B, you would set it up as {6-5, 7-8, 8-9} and simplifying gives you {1,-1,-1}
And for B-A, you set it up as {5-6,8-7,9-8}, giving you {-1,1,1}
Final solutions
A: {1,-1,-1}
B: {-1,1,1}
Think of it like this:
A:{x₁,y₁,z₁} B:{x₂,y₂,z₂}
B - A : {x₂-x₁,y₂-y₁,z₂-z₁}
A - B: {x₁-x₂,y₁-y₂,z₁-z₂}