Hi. I've been racking my brains a little longer than I anticipated on this question from section 1.2 of Velleman's "How to Prove It".
13. Use the first DeMorgan's Law and the double negation law to derive the second DeMorgan's Law.
After trying various starting combinations of P and Q and attempting to use the first DeMorgan's Law [(P and Q)' <-> P' or Q'], I've hit a brick wall in finding a workable starting point. Any and all help will be greatly appreciated. Thanks!