We have $n\lambda =2d\sin \theta$
$1\times (1.54 \times 10^{-8})=2\times d \times \sin 11.29$
$\Rightarrow d=3.933\times 10^{-8}cm$
Further $d=\large\frac{a}{\sqrt{h^2+k^2+l^2}}$
$3.933\times 10^{-8}=\large\frac{a}{1^2+0^2+0^2}$
$\therefore a=3.933\times 10^{-8}$ cm