Initial position of center of mass,

$x_{\mathrm{i}}=\frac{1\left(0\right)+1\left(10\right)}{1+1}=5\mathrm{ }\mathrm{m}$

Initial velocity of center of mass,

$v=\frac{1\left(5\right)+1\left(-3\right)}{1+1}=1\mathrm{ }\mathrm{m}$

This velocity remains constant as the internal forces during collision do not change the velocity of the center of mass. Therefore distance travelled by the center of mass in $2\mathrm{s}$ ,

$\mathrm{\Delta }x=v\mathrm{\Delta }t$

$\mathrm{\Delta }x=1\left(2\right)=2\mathrm{ }\mathrm{m}$

Final position of center of mass,

$x_{\mathrm{f}}=x_{\mathrm{i}}+\mathrm{\Delta }x$

$x_{\mathrm{f}}=5+2=7\mathrm{ }\mathrm{m}$