A plane mirror is lying in $xy$ plane. Object velocity is $v_0=(2\hat{i}-3\hat{j}+4\hat{k}) m/s$. Find the image velocity.

Components of object velocity parallel to plane mirror (or lying in $xy$ plane) remain unchanged. But, component perpendicular to plane mirror changes its direction but magnitude remains the same.

hence, the image velocity is $v_I = (2\hat{i}-3\hat{j}-4\hat{k}) m/s$