Having had just checked into the Grand Hotel, Mr. Hilbert slumped into the hotel room armchair with relief. Finally he could have some peace and quiet and solve the quaint riddle his colleague gave him! He pulled a note out from his breastpocket, and on it was simply scribbled:
Fix this equation by adding three mathematical symbols:
$$2 \space\space\space 2 \space = \space9$$ NO letters, numbers, or tampering with the equal sign!
Unfortunately, an hour and thousands of incorrect symbols later, Mr. Hilbert remained hopelessly stuck. Can you help Mr. Hilbert?