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You have to balance the charges of the ions in the compound.
Fluorine has a charge of -1, so when magnesium replaces the iron (III), the new compound is MgF2. Now all that is left to do is balance the reaction.
The sulfate group has a charge of -2, meaning lithium sulfate is Li2SO4. Potassium doesn't exist as a diatomic, so the actual reaction is:
Li + K2SO4 yields Li2SO4 + K
Also balance the above reaction.
Hope this helps.