The mass loss must represent #CO_2#. Why? Because carbonates are KNOWN to undergo thermal decomposition according to the following rxn:
#MgCO_3(s) + Delta rarr MgO(s) + CO_2(g)uarr#
And so ..........................
#"moles of carbon dioxide "-=" moles of magnesium carbonate"#
#"Moles of carbon dioxide"=(4.4*g)/(44.01*g*mol^-1)=0.0100*mol#,
which constitutes a mass of #0.0100*molxx84.31*g=8.43*g# WITH RESPECT TO #"magnesium carbonate".#
The balance, #(12.46-8.43)*g=6.03*g# WITH RESPECT TO #"magnesium oxide"#.