Set questions(discrete mathematics) Let $A$, $B$ and $C$ be sets.
Prove or disprove (with a counter example) each of the following:
(a) If $A / C = B / C$ then $A = B$.
(b) If [($A \cap C = B \cap C)\&( A / C = B/ C)$] then $A = B$.
(c) If [$(A \cup C = B \cup C)\&(A / C = B / C)$] then $A = B$. 
I understand why they are equal when I think about it visually in my head but how do I prove or disprove them formally? Like the steps required?

Hint
For proving equality, apply the definitions:

(i) $A \setminus C = B \setminus C$ iff $x \in A \setminus C \leftrightarrow x \in B \setminus C$, for any $x$

and :

(ii) $x \in A \setminus C$ iff $x \in A$ and $x \notin C$.

Note : for $A \setminus C$, note that it is not necessary that the "subtracted" set (in this case : $C$) must be included into the other one ($A$).

To disprove them, you have to find some suitable counterexamples.
