Answer
$\phi = 4.11~rad$
Work Step by Step
We can find the wave length:
$\lambda = \frac{v}{f} = \frac{330~m/s}{540~Hz} = 0.611~m$
We can find the phase difference:
$\phi = \frac{\Delta L}{\lambda}~2\pi$
$\phi = \frac{0.40~m}{0.611~m}~2\pi$
$\phi = 4.11~rad$
