Chapter P, Prerequisites - Section P.2 - Real Numbers - P.2 Exercises - Page 15: 5

$|a-b|$, $7$

Take some cases and try it out. The distance between $5$ and $3$ is $|5-3|=2$. The distance between $6$ and $-3$ is $|-3-6|=9$. We take the distance from both to $0$ and subtract to receive the distance from eachother. Applying this to our example, we have $|-2-5|=|-7|=7$.