Question

Please answer the question

Answer 1

I believe it is number iii #3

Answer 2

The angles of a triangle sum up to $180^\circ$. So, if you know two of them (let's say $\alpha$ and $\beta$), you can find the third angle $\gamma$ by subtraction:

$\alpha +\beta +\gamma = 180^\circ \iff \gamma = 180^\circ-\alpha -\beta = 180^\circ-(\alpha +\beta)$

So, in the first case, the given angles sum to $120^\circ$, so the remaining one must be $60^\circ$

In the second case, the given angles sum to $19^\circ$, so the remaining one must be $161^\circ$

In the third case, the given angles sum to $108^\circ$, so the remaining one must be $72^\circ$

In the fourth case, the given angles sum to $159^\circ$, so the remaining one must be $21^\circ$

Finally, the triangle with two sides of the same length is the one with two angles of the same measure, so the third one, which is isosceles.