(a) $n_{x} = 300Hz,\ n_{y} = \ ?$
$x =$ beat frequency $= 4\ Hz,$ which is decreasing $(4 \rightarrow 2)$ after increasing the tension of the string $y.$
Also tension of wire $y$ increasing so $n_{y} \uparrow (\because n \propto \sqrt{T})$
Hence $n_{x} - n_{y} \uparrow = x \downarrow \ \ \ \rightarrow Correct$
$n_{y} \uparrow - n_{x} = x \downarrow \ \ \rightarrow Wrong$
$\Rightarrow n_{y} = n_{x} - x = 300 - 4 = 296Hz$