$\left(\frac{u}{3}\right)^2=u^2-2 a x$

$2 a x=u^2-\frac{u^2}{9}$

$2 a x=\frac{8 u^2}{9}.........(1)$

Similarly from starting

$v^2=u^2+2 ax$

$0= u ^2-2 ax _2$

$2 ax _2= u ^2..........(2)$

$By (1) /(2)$

$\frac{ x }{ x _2}=\frac{8}{9}$

$\frac{24}{ x _2}=\frac{8}{9}$

$x _2=27\,cm$