$($a$)$ If the sixth$-$order maximum for pure$-$wavelength light falling on a double slit is at an angle of $39\mathrm{.}2,$ at what angle $($in degrees$)$ is the seventh$-$order maximum?

$-$

$($b$)$ What is the angle $($in degrees$)$ of the fourth minimum?

$$

$($c$)$ What is the highest$-$order maximum possible here?

$$