a
\(\begin{gathered}
{x_{CM}}\, = \,\,\,\frac{{0\, \times \,\,m\,\, + \,\,a\,\, \times \,\,m\,\, + \,\,\frac{a}{2}\,\, \times \,\,m}}{{m\,\, + \,\,m\,\, + \,\,m}}\,\,\, = \,\,\frac{a}{2} \hfill \\
{y_{CM}}\, = \,\,\,\frac{{0\,\, \times \,\,m\,\, + \,\,0\,\, \times \,\,m\,\, + \,\,\frac{{a\sqrt 3 }}{2}\,\, \times \,\,m}}{{m\,\, + \,\,m\,\, + \,\,m}}\,\,\, = \,\,\frac{{a\sqrt 3 }}{6} \hfill \\
\end{gathered} \)