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How would I go about proving this without a truth table?

$[(p \lor q) \implies r ] \implies [ \neg r \implies (\neg p \land \neg q)]$

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    What are the rules in the proof system you're supposed to use?2012-10-04
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    I understand how (p or q)→r = (~p AND ~q) → ~r due to De Morgan's Law How do you prove that the reverse of an implication is true? So that the reduction equals what I am trying to prove.2012-10-04

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