These types of questions look weird, but when they give you the variable or "j", it is quite nice. The word "evaluate" is used when the expression does not have an equal sign. All you really have to do is simplify it.
J=12 means "the letter "J" is representing 12. The letter "J" could represent anything in this question, but for this question we are told that "j' equals the number 12.
Simply substitute 12 for "J" in the expression and now you have 12 divided by 4. Then you could say the expression becomes 3 if you simplify 12 over 4 which is the same as 12 divided by 4.
A real life example of this question could be, "4 packs of jewels on a game cost $12.
Not the best explanation, but I hope it helps.