Answer
The amplitude of the net wave is $~~21~nm$
Work Step by Step
We can find the amplitude of the net wave:
$s_m' = 2~s_m~cos~\frac{\phi}{2}$
$s_m' = (2)~(12~nm)~cos~\frac{\pi/3}{2}$
$s_m' = (2)~(12~nm)~cos~\frac{\pi}{6}$
$s_m' = 21~nm$
The amplitude of the net wave is $~~21~nm$
