Video could finish shortly after 3:40. Factorials are defined for non-negative integers only and -3 is negative, so _p_ = -3 is automatically excluded as a solution. Also a true-false test for _p_ = 4 is not necessary since all the applied operations are equvialent, i.e. they do not change the set of solutions for our equation. However, it depends on the mathematics curriculum in your country...

Sorry, I have to say it was clumsy and stupid.