Problem 27.17

27.17.
For destructive interference, we have that
$\sin \theta = m \frac{\lambda}{a}$
Here we want the case when m = 1. So that leaves us with
$\sin \theta = .13889$
$\theta = 7.98{}^{\circ}$
To find the distance along the wall we employ a little trigonometry.
$d = L \tan \theta$
$d = (6.5 m) \tan 7.98{}^{\circ}$
d = .912 m