MgO + CaS —> MgS + CaO
The equation is already balanced as there are the same number of atoms on both sides. (there is 1 Mg on the right hand side and 1 Mg on the left hand side, 1 Ca on the right hand side and 1 Ca on the left handside, 1 S on the right hand side and 1 S on the left handside and 1 O on the right hand side and 1 O on the left side)