(a) For which values of $a$ does $x+a=0$ have a solution in $\mathbb{Z}_{5}$ ?
(b) For which values of $a$ and $b$ does $x+a=b$ have a solution in $\mathbb{Z}_{6} ?$
(c) For which values of $a, b,$ and $m$ does $x+a=b$ have a solution in $\mathbb{Z}_{m}$ ?