Can anyone construct a formal proof for this argument using an equivalence

Statement Reason
1. ~ s Λ ~ r - premise
2.~r -C.S. (1)
3. r V ( m → n ) -premise
4.m → n -D.S. (2,3)
5. ~ ( d V n ) -premise
6.~m Λ~n -con. Neg. (4)
7.~ m -C.S. (6)