????????
      +---------
GNLNF | MRXRHHXU
       -GNLNF
        =====
        GMLMRH
       -GMGHRM
        ======
          NRTMX
         -NPGUH
          =====
           PRQUU
          -PTPQP
           =====
           ?????

base: 13
letters: FGHJLMNPQRTUX
difficulty: medium
missing: J