Prove that $a^2 \equiv 4 \mod 3^m$ if and only if $a \equiv 2 \mod 3^m$ or $a \equiv −2 \mod 3^m$
If $x^2 \equiv y^2 \pmod {p^r}$, where $p$ is an odd prime not dividing $x$ or $y$ then $x \equiv \pm y \pmod {p^r}$