"H"_3"PO"_4" + Mg(OH)"_2rarr"Mg"_3"(PO"_4)_2 + "H"_2"O"
Balance the magnesium first. There are three magnesium ions on the right side and only one on the left side. Add a coefficient of 3 in front of "Mg(OH)"_2"..
"H"_3"PO"_4" + 3Mg(OH)"_2rarr"Mg"_3"(PO"_4)_2 + "H"_2"O"
Next balance the "PO"_4".
There are two "PO"_4" ions on the right side and one on the left. Place a coefficient of 2 in front of "H"_3"PO"_4".
"2H"_3"PO"_4" + 3Mg(OH)"_2rarr"Mg"_3"(PO"_4)_2 + "H"_2"O"
Next balance the "H". There are twelve hydrogens on the left side and two on the right. Place a coefficient of 6 in front of "H"_2"O" on the right side.
"2H"_3"PO"_4" + 3Mg(OH)"_2rarr"Mg"_3"(PO"_4)_2 + "6H"_2"O"
Balance the "O". There are fourteen oxygen atoms on both sides of the equation, so it is already balanced.
Balanced Equation
"2H"_3"PO"_4" + 3Mg(OH)"_2rarr"Mg"_3"(PO"_4)_2 + "6H"_2"O"