The answer would be -7. Explanation: You would do the opposite of addition so you would subtract 42 on both sides so on the right side it would get rid of the 42, then you would subtract it from the 21 so what's 21-42? The answer would be -21. Then you would divide 3 on each side so 3 divided by 3 would cancel that. So then you would do 3/-21 which would be -7. (: