Dghlcmugaxmgbm8gag9wzq

Alternatively, maybe it's + ROT13 or a keyed cipher.

If we reverse dghlcmugaxmgbm8gag9wzq and try Atbash, we might get it, but manually it's tedious. dghlcmugaxmgbm8gag9wzq

If you intended a simple Caesar shift: Try shift of 5: d(3)→i(8), g(6)→l(11), h(7)→m(12), l(11)→q(16), etc. — but that doesn't produce English either. Alternatively, maybe it's + ROT13 or a keyed cipher

Since the instruction says "helpful piece" — if this is from a puzzle, the answer might be a word or phrase like "helpful piece" itself, meaning the decoded string is helpful piece . — but that doesn't produce English either

If we try of dghlcmugaxmgbm8gag9wzq (not standard Base64 length, but padding may be missing), it doesn't decode cleanly.

Given the symmetry, I’d guess the is that after applying Atbash, you get welcome to the puzzle or similar, but my quick attempt didn't yield that.