Answer
a) $MgSO_4$
b) $FePO_4$
Work Step by Step
a) Possible salt formations: $NaNO_3$ and $MgSO_4$, nitrates are soluble but $MgSO_4$ isn't soluble.
b) Possible salt formations: $KCl$ and $FePO_4$, $KCl$ is soluble but $FePO_4$ is not.
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