My son has an assignment, and he doesn't know what
l-3l means. Can someone explain what it is?

That is the absolute value of -3, when you take the absolute value of something, you would get rid of everything in front of the number. The absolute value of -3 is 3.
Hope this helps.

It absolute value it is asking what is the absolute value meaning-opposite which is -3 turns to a positive number so it would be |3|