\displaystyle \displaystyle \mathbf{r}_g = \frac{1\textrm{kg}(3,5)\textrm{m} + 2\textrm{kg}(-4,6) + 4\textrm{kg}(3,-6)\textrm{m}}{1\textrm{kg}+2\textrm{kg}+4\textrm{kg}} = \frac{(7,-7)}{7}\textrm{m} = (1,-1)\textrm{m}
