ANSWER
MN = 7
EXPLANATION
Let's draw a diagram first:
Since all the points are collinear we can use the segment addition postulate:
[tex]LM+MN=LN[/tex]
Replacing with the values we know:
[tex]13+MN=20[/tex]
And solving for MN:
[tex]MN=20-13=7[/tex]
We have that the length of segment MN is 7.
