Incorrect Answer.
To see that the sequent
p 
q 
r, p, r 
r cannot be valid, we only have
to find an assignment of truth values to
p, q, and r, such that r is false whereas the three formulas
p q r, p, and q are true.
But these constraints determine that p, q, and r must have truth value F and
these constraints are indeed satisfied, given that
assignment of truth values. By soundess of the natural deduction calculus
for propositional logic, the sequent above cannot be valid, meaning that it
cannot have a proof.
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