Modular homework problem Show that:
$$[6]_{21}X=[15]_{21}$$
I'm stuck on this problem and I have no clue how to solve it at all.

One way is just to try all the choices for $X$, the integers from $0$ to $20$.  That isn't very many.
Another is to look for representatives in the class of $15$, which are numbers of the form $15+21k$, for one that is a multiple of $6$.
