???????
      +--------
PWWXR | FHMRFRF
       -FQNRY
        =====
         RQMYR
        -FQNRY
         =====
         PTGFWF
        - MXMFX
         ======
         ??????

base: 13
letters: FGHMNPQRTUWXY
difficulty: medium
missing: U