The centre of mass of the remaining object is

$x_{\mathrm{cm}}=\frac{m_1{x}_1-m_2{x}_2}{m_1-m_2}=\frac{\sigma A_1{x}_1-\sigma A_2{x}_2}{\sigma A_1-\sigma A_2}$

where $m_1$ and $A_1$ are the mass and area of the total disc and $m_2$ and $A_2$ are the mass and area of the cut-out portion. $\sigma$ is the mass per unit length of the disc.

$A_1=\mathrm{\pi }(3R{)}^2$, $A_2=\pi R^2$

$x_1=0$, $x_2=2R$

$\Rightarrow x_{\mathrm{cm}}=-R/4$