Answer
$31.53\mu m$
Work Step by Step
We know that
$\theta_{min}=\frac{1.22\lambda}{1.36D}$
$\theta_{min}=\frac{0.897(540\times 10^{-9}m)}{4.3\times 10^{-3}m}$
$\theta_{min}=11.26\times 10^{-15}rad$
Now $y=Ltan\theta_{min}$
We plug in the known values to obtain:
$y=(28\times 10^{-2}m)tan(11.26\times 10^{-5}rad)$
$y=31.53\mu m$