Answer
$a=0.570nm $
Work Step by Step
We know that:
$a=\sqrt2 \space d$
We also know that:
$d=\frac{\lambda}{2sin\theta}$
So,
$a=\sqrt2 \space \frac{\lambda}{2sin\theta} $
We plug in the known values to obtain:
$a=\sqrt2 \space \frac{0.260nm}{2sin18.8^{\circ}}=0.570nm $
