????????
      +---------
26F3D | 199BBE05
       -26F3D
        =====
        3E14AE
       -306395
        ======
         5AD400
        -AF9ED5
         ======
          267E95
         -6BA815
          ======
          ??????

base: 16
letters: 0123456789ABCDEF
difficulty: challenging
missing: C