Answer
$154\mu m$
Work Step by Step
We can find the required slit separation as follows:
$y=Ltan\theta$
$\implies \theta=tan^{-1}(\frac{y}{L})$
We plug in the known values to obtain:
$\theta=tan^{-1}(\frac{11.5\times 10^{-3}}{1.40})$
$\theta=tan^{-1}(0.00821)=0.4706$
Now $d=\frac{m\lambda}{sin\theta}$
We plug in the known values to obtain:
$d=\frac{2(632.8\times 10^{-9})}{sin(0.4706)}$
$d=154153.47\times 10^{-9}m$
$d=154\times 10^{-6}m=154\mu m$
