What does a bar above a letter mean in set theory?

The bar probably means every thing that is NOT in the set (I mean not in the set that does not have the bar)

 

The bar is used to indicate the complementary set.

write an equation, in any form, of the line through (3,2) AND (-4,5)?

Well, why don't you ask the teacher that teaches you Set Theory?!. That's why he/she is there for. There is NO SHAME in asking your teacher.