????????
      +---------
QQHEN | QYHTFRHY
       -QQHEN
        =====
         EYHHR
        -EPEFT
         =====
          HYFWH
         -HQJGY
          =====
           WFFEY
          -EPEFT
           =====
           ?????

base: 13
letters: EFGHJNPQRTUWY
difficulty: hard
attributes: system of equations
missing: U