Existence & uniqueness of solutions of linear congruences $\ cx\equiv b\pmod m$

Hint $ $ If $ \,x,y\,$ are solutions then $ \ cx\equiv b\equiv cy\pmod{\! m}\ $ so, defining $ \ d=(m,c)\,$ we have

$$ \,m\,|\,c\,(x\!-\!y) \iff \color{#0a0}{\frac{m}d}\,\bigg|\,\color{#0a0}{\frac{c}d}\,(x\!-\!y)\color{#C00}{\iff} \frac{m}d\,\bigg|\ x\!-y,\ \ {\rm by} \ \ \left(\color{#0a0}{\frac{m}d,\frac{c}d}\right)\color{#90f}{= \frac{(m,c)}d} = 1\qquad$$

Remark $\ $ The final $ $ '$\color{#C00}{\!\!\iff}\!\!$' $ $ employs $\rm\color{#0a0}{Euclid's\ Lemma}$ and the $\rm\color{#90f}{\rm distributive\ law}$ for GCDs.

Alternatively $\ $ Bezout yields: $ \, d = (c,m) = j\:\!c+k\:\!m,\,\ j,k\in\Bbb Z.\,$ Let $ \,z = x-y.\,$ Then

$\!\bmod m\!:\ \color{#0a0}{cz\equiv 0\equiv mz}\ \Rightarrow\ dz = (c,m)z = j\:\!\color{#0a0}{cz}+k\:\!\color{#0a0}{mz \equiv 0},\,$ so $ \ m\,|\,dz\,\Rightarrow\,m/d\,|\,z$