Answer
(a) $L = 0$
(b) $L = 3.46 ~\hbar$
Work Step by Step
(a) In the $6s$ state, $l = 0$
We can find the angular momentum:
$L = \sqrt{l~(l+1)}~\hbar$
$L = \sqrt{(0)~(0+1)}~\hbar$
$L = 0$
(b) In the $4f$ state, $l = 3$
We can find the angular momentum:
$L = \sqrt{l~(l+1)}~\hbar$
$L = \sqrt{(3)~(3+1)}~\hbar$
$L = 3.46 ~\hbar$
