Answer
$d_{avg} = 3.3 nm$
Work Step by Step
To find the average distance, use the equation
$d_{avg} = n^{1/3}$
Take $n_o$ from (a) which is $2.7 \times 10^{25} m^{-3}$
So:
$d_{avg} = (2.7 \times 10^{25} m^{-3})^{1/3}$
$d_{avg} = 3.3 nm$