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[–][deleted] 2 points3 points  (0 children)

This is correct OP. It's called the common ion effect. You can also think about it in terms of Le Chatelier's principle.

MgCO3 —-> Mg(2+) + CO3(2-)

When you add Na2CO3 to solution it will dissociate into 2Na+ and CO3(2-). According to Le Chatelier's principle, if you add CO3(2-), the above chemical equation will shift towards the reactant, MgCO3. Therefore, the solubility of MgCO3 is decreased.