????????
      +---------
HXJMJ | PXTWFXXX
       -HXJMJ
        =====
        GMWWMX
       -GQHYWM
        ======
         HFEHMX
        -HTXFJQ
         ======
          GPJPGX
         -GPTGQW
          ======
          ??????

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