$l_{1}+x=\frac{\lambda}{4}=22.7 \ldots \ldots( I )$

Second resonance,

$l_{2}+x=\frac{3 \lambda}{4}=70.2 \ldots \ldots$ $(II)$

Third resonance,

$l_{3}+x=\frac{5 \lambda}{4} \ldots \ldots$ $(III)$

From equations $(I)$ and $(II),$

$x=\frac{l_{2}-3 l_{1}}{2}$

$x=\frac{70.2-68.1}{2}$

$x=1.05 cm$

Now from equations $(II)$ and $(III),$

$\frac{l_{3}+x}{l_{1}+x}=5$

$\frac{l_{3}+1.05}{68.1+1.05}=5$

After simplification,

$l_{3}=117.7 cm$