g => (g or h) => (s and t) => t => (t or u) => (c and d) => c.
We are given premises:
# (g or h) -> (s and t) # (t or u) -> (c and d) We would like to derive g -> c.
If we assume g (the antecedent in the conclusion) we have the following derivation: # g (assumption) # g or h(weakening) # s and t (premise 1 (modus ponens)) # t(weakening) # t or u (weakening) # c and d (premise 2 (modus ponens)) # c (weakening)
So, assuming g we can derive c, i.e. g -> c