In an experiment to determine the velocity of sound in air at room...

Wavelength of wave $⇒ \lambda = \dfrac{v}{f} = \dfrac{336}{400} = 84cm$

At first resonance

$\dfrac{\lambda}{4} = l + e ⇒ \dfrac{84}{4} = 20 + e$, where, $e$ is end correction.

$⇒ e = 1$
So third resonance length

$5\dfrac{\lambda}{4} = {l}_{2} + e$

$5\left(21\right) = {l}_{2} + 1$

${l}_{2} = 104cm$