what does I 3-9 I mean? And how would you calculate it?

|3 - 9| means an absolute value.

so like |3| is 3. |-9| is 9. It is the opposite number, unless it is already a positive.

Your answer is |3-9| = 3 + 9 = 12.

| 3-9 | = 12

FAIL

Kinda....sorta Obscure..... you must first do the calc between the abs value bars and THEN make it positive

|3-9| = |-6| = 6