a
First resonance,

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

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,

\(l_{3}=117.7 cm\)