????????
      +---------
76C63 | 799C2C4B
       -76C63
        =====
         328AC
        -1004C
         =====
         777B54
        -754AB4
         ======
           2975B
          -04765
           =====
           ?????

base: 14
letters: 0123456789ABCD
difficulty: hard
missing: D