Answer
92 cm.
Work Step by Step
The angular size of the first dark ring is given by $sin \theta_1=1.22\frac{\lambda}{D}$.
$$sin \theta_1=1.22\frac{620\times10^{-9}m }{7.4\times10^{-6}m }=0.1022$$
$$ \theta_1=5.867^{\circ}$$
The radius of the ring on the screen is $(4.5m)(tan \theta_1)=0.462 m$, and the diameter is twice that.
