Fringe width = $6 mm = \frac {\lambda D}{d} = \frac {\lambda \times 10}{1\times 10^{-3}}$
$\Rightarrow \lambda = \frac {6 \times 10^{-3} \times 10^ {-3}}{10} = 6 \times 10^{-7} = 600 nm$
$\therefore x = 600$