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

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

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

Similarly from starting

\(v^2=u^2+2 ax\)

\(0= u ^2-2 ax _2\)

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

\(By (1) /(2)\)

\(\frac{ x }{ x _2}=\frac{8}{9}\)

\(\frac{24}{ x _2}=\frac{8}{9}\)

\(x _2=27\,cm\)