Answer:
2, 3, 1, 6.
Explanation:
2H3PO4 + 3Mg(OH)2 --> Mg3(PO4)2 + 6H2O
This is a neutralisation reaction because it is the reaction between acid and base to give a salt and water.
The RHS is the first to be balanced, Mg atom is first balanced, followed by the P atom.
This is because the excess O and H atoms can be balanced on the LHS.