I think it's commutative property of addition (you can change order of addition but in the end it would be the same)
The vector AB is found by subtracting the coordinates of point B from the coordinates of point A => (8, 3, 4) - (5, 7, - 2)
(8, 3, 4) - (5, 7, - 2) = (3, - 4, 6)
=>
a) a 3 x 1 column =
| 3 |
| -4 |
| 6 |
b) xi + yj + zk
=> 3i - 4j + 6k
B, x is less than or equal to one