When you have one writing letter what do you have? Here's a hint, there's one kanji in this word that consists of one writing letter (aka a straight line).
This is a jukugo word, which usually means on'yomi readings from the kanji. Since 文 has two on'yomi readings, here's a mnemonic to help you remember which one to use:
Straight lines are for monsters (もん). Only a monster would ever draw a perfectly straight line. You're no monster, so no straight lines for you!
to make a straight line
to purse (one's lips)
to stand in a straight line
The girl got angry and her mouth pursed into a straight line.
The salmon has a straight line cut across it.
Quite a few drones were flying in a straight line.