sort out

(verb) to try and understand the cause of a problem

Example Sentence:

The roommates tried for four hours to sort out why their apartment smelled so bad. Then they realized it was because it was dirty.

A: I can’t sort out why I keep getting this math problem wrong.
B: Here, let me help you.

Boyfriend: Why are you so upset?
Girlfriend: You should be able to sort it out . You had sex with my sister!

Related:

figure it out