\({\overrightarrow a _C} = a\hat i\,\,\,\,\,\,\,\,\,\,\,\,\,;\,\,\,\,\,\,\,\,\,\,\,\,{\overrightarrow a _D} = - a\hat j\)
\({\overrightarrow a _{cm}} = \frac{{{m_a}{{\overrightarrow a }_a}a + {m_b}{{\overrightarrow a }_b} + {m_c}{{\overrightarrow a }_c} + {m_d}{{\overrightarrow a }_d}}}{{{m_a} + {m_b} + {m_c} + {m_d}}}\)
\({\overrightarrow a _{cm}} = \frac{{ - ma\hat i + 2ma\hat j + 3ma\hat i - 4ma\hat j}}{{10m}}\)
\( = \frac{{2ma\hat i - 2ma\hat j}}{{10m}} = \frac{a}{5}\hat i - \frac{a}{5}\hat j = \frac{a}{5}\left( {\hat i - \hat j} \right)\)