Answer
The z-coordinate of the center of mass is $~~1.40~cm$
Work Step by Step
By symmetry, the z-coordinate of the center of mass is:
$z_{com} = \frac{d_2}{2}$
$z_{com} = \frac{2.80~cm}{2}$
$z_{com} = 1.40~cm$
The z-coordinate of the center of mass is $~~1.40~cm$.
