????????
      +---------
FMJNW | FWEFEFYX
       -FMJNW
        =====
         JRXFF
        -FMJNW
         =====
         MFJXRY
        -MYYXXJ
         ======
          XEHLTX
         -XHHFJR
          ======
          ??????

base: 13
letters: EFHJLMNQRTWXY
difficulty: challenging
missing: Q