Answer
$L = 0.35~m$
Work Step by Step
We can find the wavelength as:
$\lambda = \frac{v}{f}$
$\lambda = \frac{350~m/s}{250~Hz}$
$\lambda = 1.4~m$
We can find the length of this tube as:
$\lambda = 4L$
$L = \frac{\lambda}{4}$
$L = \frac{1.4~m}{4}$
$L = 0.35~m$