Answer
$\rm 4\;mm,0\;mm,-4\;mm$
Work Step by Step
At point a, there is constructive interference.
Thus, the net displacement of the medium is
$$D_a=2+2=\color{red}{\bf 4}\;\rm mm$$
At point b, there is destructive interference.
Thus, the net displacement of the medium is
$$D_b=2+(-2)=\color{red}{\bf 0}\;\rm mm$$
At point c, there is constructive interference as well.
Thus, the net displacement of the medium is
$$D_c=-2+(-2)=\color{red}{\bf -4}\;\rm mm$$