Assume ~(a and ~b)
By De Morgan's Law*, ~(a and ~b) is equivalent to ~a or ~(~b), which can be written ~a or b .
Now suppose ~b.
Since we assumed the equivalent of ~a or b as our premise, then supposing ~b implies ~a.
*Note: I'm guessing that De Morgan's Law ( ~(p and q) = ~p or ~q) was covered in a previous question. Check using a truth table to verify for yourself that this is true.
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