Open a dial safe.

You should be able to get this pretty early in the game. During Leon's campaign one of the very first puzzles you come across will be in the West Office. You will find a note on your desk explaining that you need to crack the two combination locks using the first letters of the other officers' names.
Check the two rows of desks to learn the names of the 6 officers: Marvin, Elliot, George, Rita, David and Neil.
The lock on the left opens with the code NED and the lock on the right opens with MRG.

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Achievement is worth 20 Gamerscore.