Answer
$20\mu m$
Work Step by Step
From part a, we know we have a slit width of $a = 5.0\mu m $. From the given graph, we note that the fourth bright fringe ($m_2=4$) is on the first minima and is located at $\theta=5.0^{\circ}$.
We can now use equation (36-25) to find the slit separation, $d$;
$d \sin \theta = m_2 \lambda $ where $m=1,2,3,....$ (maxima)
$d = m_2 \lambda /\sin \theta$
$d = 4 *0.440\mu m /\sin 5.0^{\circ}$
$d=20\mu m$
