what is the length of $overline{mn}$?

Explanation:

Step1: Identify coordinates

The coordinate of $M$ is $- 523$, and the coordinate of $N$ is $-521$.

Step2: Use distance formula

The distance between two points $a$ and $b$ on a number - line is $|a - b|$. Here, the length of $\overline{MN}=|-521-(-523)|$.

Step3: Simplify the expression

$|-521 + 523|=|2| = 2$.

Answer:

$2$