Answer
Please see the work below.
Work Step by Step
(a) We know that the condition for the bright fringes in a two-slit experiment is given as
$ dsin\theta=m\lambda $
$\implies dsin\theta=4\lambda $
(b) The condition for the dark fringe is given as
$ dsin\theta=4\lambda $
$\implies dsin\theta=(m-\frac{1}{2})\lambda $
$\implies dsin\theta=(3-\frac{1}{2})\lambda=\frac{5}{2}\lambda $
