Answer
$3.6\times10^{-6}m$
Work Step by Step
For constructive interference to occur, the path difference is a whole multiple of the wavelength. This is stated in equation 24–2a.
$$d sin \theta = m \lambda $$
Apply this to the third order bright fringe.
$$d = \frac{m \lambda}{ sin \theta }$$
$$d = \frac{3(610\times10^{-9}m)}{sin 31^{\circ}}$$
$$d = 3.6\times10^{-6}m $$
