Magnesia (MgO(s)) is used for fire brick, crucibles and furnace linings because of its high melting point. It is produced by decomposing magnesite (MgCO3(s)) into at around 1200oC

Question

Write a balanced equation for the magensite decomposition being sure to include states for the species.

I thought it was:
MgCO3(s) ...> MgO (s) + CO2(g)

but it was marked wrong.

help..thank you.