SOLUTION: (M ∨ R) ⊃ ∼K (G ⊃ ∼K) ⊃ C G ⊃ (M ∨ R) /C

1.  (M or R) -> ~K
2.  (G -> ~K) -> C
3.  G -> (M or R)           |  C

 4.  Assume ~C 
      5.  ~(G -> ~K)              2,4 Modus Tollens
      6.  G and K                 5 Definition of Implication
      7.  G                       6 Simplification
      8.  K                       6 Simplification
      9.  ~(M or R)               1,7 Modus Tollens
     10.  ~G                      3,9 Modus Tollens
11.  ~C -> (G and ~G)        4,7,10
12.  Therefore C             11 Tertium non datur