The contrapositive of (p∨q)→r is
r→(p∨q)
~r→(p∨q)
~r→(~p∧~q)
p→(q∨r)
Contrapositive of p→q is ~q→∼p
Therefore, contrapositive of (p∨q)→r is
~r→~(p∨q) ≡∼r→(~p∧~q)