(a) The distance between zeroth order maxima and second order minima is
$y_{1} = \frac{\beta}{2} + \beta = \frac{3}{2}\beta$
$= \frac{3}{2} \times 0.2\ mm = 0.3\ mm$
$\therefore$ The distance of second maxima from point P is
$y = (4.8 + 0.3)mm = 5.1\ mm$