Where is a grasshopper's hearing organs?

That depends on the kind of grasshopper. If you mean "Long-horned grasshoppers", what some people call "katydids", they are in slits near the joint that looks like the "knee" of the front leg.

If you mean short-horned grasshoppers or 'Locusts", it is a larger "tympanum" or "eardrum" on each side of the base of the abdomen, the "belly" part of the insect.