SOLUTION
Make a truth table for ~[~(p→q)→~r]
| p | q | r | ~p | ~q | ~r | p→q | ~(p→q) | ~(p→q)→~r | ~[~(p→q)→~r] | |
| T | T | T | F | F | F | T | F | T | F | |
| T | T | F | F | F | T | T | F | T | F | |
| T | F | T | F | T | F | F | T | F | T | |
| T | F | F | F | T | T | F | T | T | F | |
| F | T | T | T | F | F | T | F | T | F | |
| F | T | F | T | F | T | T | F | T | F | |
| F | F | T | T | T | F | T | F | T | F | |
| F | F | F | T | T | T | T | F | T | F |