Answer
$0.221\;\rm mm$
Work Step by Step
We know, in a double-slit experiment, that
$$\Delta y=\dfrac{\lambda L}{d}$$
where $\Delta y$ is the distance between two bright fringes, $L$ is the separation distance between the slit and the screen, and $d$ is the distance between the two slits.
Thus,
$$d =\dfrac{\lambda L}{ \Delta y}$$
Plugging the known;
$$d =\dfrac{(589\times 10^{-9}) (1.50)}{(4\times 10^{-3})}=\bf 0.000221\;\rm m$$
$$d =\color{red}{\bf 0.221}\;\rm mm$$
