We know already that desk A and desk B are on the same column, since both coordinates' x factors are 7. So all we need to do now is find the differeance of 2 and -8. So, we get the [very simple] equation 2 - (-8) = d. Now, when you subtract a negative, it's pretty much just adding, so we can simplify this equation to 2 + 8 = d, and we all know 2 + 8 = 10. So the distance between the two desks is 10 units.
Hope this helps! If there is anything wrong feel free to point it out.
