Need to use Fermat's Little Theorem (Let $p$ be a prime number and let $a$ be an integer. Then $a^p = a \mod p$. If $p$ does not divide $a$ then $a^p-1 \equiv 1 \mod p$.) $154$ is not prime, but $154 = 22\cdot 7$ and $23$ is prime, so $a^{22} \equiv 1 \mod 23$. $a^{154} \equiv a^{22} \cdot a^7 \equiv 1 \cdot 6 \equiv 6 \mod 23$.
Not sure how to proceed further. Would be grateful for any help.