Answer
$(4\;\rm cm,\;3.3\;cm)$
Work Step by Step
To find the coordinates of the center of the mass of the given object, we need to find $X_{cm}$ and $Y_{cm}$.
$$X_{cm}=\dfrac{m_Ax_A+m_Bx_B+m_Cx_C}{m_A+m_B+m_C}$$
Plugging the known;
$$X_{cm}=\dfrac{100 (0)+200(0)+300(10)}{100+200+300}=\color{blue}{\bf5}\;\rm cm$$
And
$$Y_{cm}=\dfrac{m_Ay_A+m_By_B+m_Cy_C}{m_A+m_B+m_C}$$
Plugging the known;
$$Y_{cm}=\dfrac{100 (0)+200(10)+300(0)}{100+200+300}=\color{blue}{\bf 3.3}\;\rm cm$$
Therefore, the coordinates of the center of mass of the three masses is
$$(4\;\rm cm,\;3.3\;cm)$$
