Answer
Argument form is modus tollens.
Argument is valid
Conclusion is true since premises are true.
Work Step by Step
Modua tollens:
$$\displaylines{p \to q \cr \neg q \cr} $$$$- - - $$$$\therefore \neg p$$
Let us assume :
p="George have eight legs"
q="George is a spider".
Then we can write the given argument as
$$\displaylines{\neg p \to \neg q \cr q \cr} $$$$- - - $$$$\therefore p$$
$q\equiv \neg(\neg q)$
This can also be written as:
$$\displaylines{\neg p \to \neg q \cr \neg(\neg q) \cr} $$$$- - - $$$$\therefore \neg(\neg p)$$
Then the conclusion is true according to modus tollens rule.
