c
\({m_1} = 7\ gm\), \({m_2} = 4\ gm\), \({m_3} = 10\ gm\) and  \(\overrightarrow {{r_1}}  = (\hat i + 5\hat j - 3\hat k),\,\,{r_2} = (2i + 5j + 7k),\) \({r_3} = (3\hat i + 3\hat j - \hat k)\)

Position vector of center mass \(\overrightarrow {\,r}  = \frac{{7(\hat i + 5\hat j - 3\hat k) + 4(2\hat i + 5\hat j + 7\hat k) + 10(3\hat i + 3\hat j - \hat k)}}{{7 + 4 + 10}}\)\( = \frac{{(45\hat i + 85\hat j - 3\hat k)}}{{21}}\)  

\(\overrightarrow {r\,}  = \frac{{15}}{7}\hat i + \frac{{85}}{{21}}\hat j - \frac{1}{7}\hat k\).

So coordinates of centre of mass \(\left[ {\frac{{15}}{7},\,\frac{{85}}{{21}},\,\frac{{ - 1}}{7}} \right]\)