????????
      +---------
RTRUL | TTTMGMXT
       -RTRUL
        =====
        RQRRXM
       - HWRFH
        ======
          XHUPX
         -XPXFN
          =====
           LQJNT
          -JHNRG
           =====
           ?????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: medium
missing: Y