Please explain why the third answer is correct. Thank you.
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We have $\displaystyle \widehat{RQS}=\widehat{RSQ}=\alpha$
$\displaystyle \widehat{TSU}=\widehat{TUS}=\beta$
If we know both $\displaystyle \alpha$ and $\displaystyle \beta$ then we can find x. For this we have:
$\displaystyle \alpha+x+\beta=180^{\circ}$ and
$\displaystyle 180^{\circ}-2\alpha+180^{\circ}-2\beta=90^{\circ}$
and we get $\displaystyle x=45^{\circ}$
But if we know just $\displaystyle \alpha$ (or just $\displaystyle \beta$) then we can't find x.