what does it mean by integer/ m integer?
im asked to find all the units of integer/ 12 integer and im not sure what does that mean.
More to the point,  does not have an inverse (and so is not a unit) because = . Of course, that is a direct result of the fact that gcd(2, 12)= 2 not 1.
, , and  are units but you are missing one obvious one. Remember that the set of all units of a ring form a group with multiplication.