How can I use modus ponens or modus tollens to produce valid arguments?

Question

I know this one is:

$(1)$ If logic is easy, then I am a monkey’s uncle.

I am not a monkey’s uncle.

∴ ?

My answer: $\therefore$ Logic is not easy.

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(2) Can someone help me with this one?

If this figure is a quadrilateral, then the sum of its interior angles is 360◦.

The sum of the interior angles of this figure is not 360◦.

∴ ?

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(3) Another which has me stuck:

If they were unsure of the address, then they would have telephoned.

?

∴They were sure of the address.

Answer 1

All three are examples of modus tollens.

$P \rightarrow Q$

$\lnot Q$

$\therefore \lnot P$.

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In the second, $P$ = this figure is a quadrilateral, $Q$ = then the sum of its interior angles is 360◦.

$\lnot Q$ = The sum of the interior angles of this figure is not 360◦.

So we conclude, by modus tollens, that $\lnot P$: This figure is not a quadrilateral.

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In the third, the missing line will be the negation of "they would have telephoned."

Answer 2

It's completely analagous. The first one is the figure is not a quadrilateral, the second one would be they didn't telephone. ((If A then B) & not B) => not A, plug in various phrases for A and B.