HELPPP pleaseee, I'm in fourth and I don't understand

So,

We can visualize skydivers A, X, and B as corresponding to three coordinates -2.5, 0, and 0.5, respectively.

To find the distance from A to B, we can just take the absolute value of the different between their coordinates.

|-2.5 - 0.5| <-- this is the expression

|-2.5 - 0.5| = |-3| = 3 <-- this is the distance