The radius of electron's second stationary orbit in Bohr's atom is...

As we know, radius of hydrogen like atom $r ∝ \dfrac{{n}^{2}}{Z}$.

Therefore,

$\dfrac{{r}_{2}}{{r}_{3}} = {\left(\dfrac{{n}_{2}}{{n}_{3}}\right)}^{2}$

$⇒ \dfrac{R}{{r}_{3}} = {\left(\dfrac{2}{3}\right)}^{2}$

$⇒ {r}_{3} = \dfrac{9}{4}R$

$= 2 . 25R$