Name of challenge suggests it is either base64 or reversed.
We are given the string bellow. If we try to decode from base64 it will not work, however if we reverse the string:
Reverse this:
mxWYntnZiVjMxEjY0kDOhZWZ4cjYxIGZwQmY2ATMxEzNlFjNl13X

Into this:
X31lNjFlNzExMTA2YmQwZGIxYjc4ZWZhODk0YjExMjViZntnYWxm

Decode from base64
_}e61e711106bd0db1b78efa894b1125bf{galf

reverse again and get the flag

flag{fb5211b498afe87b1bd0db601117e16e}