Answer: 
Explanation: A double displacement reaction is one in which exchange of ions take place.
According to the law of conservation of mass, mass can neither be created nor be destroyed. Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.

Answer:
Which one? 1, 2, 3, 4 ,5, 6, or 7?
Explanation:
Answer:
12 more oxygen
Explanation:
An atom is the smallest particles that can take part in a chemical reaction
The given compounds are:
3Mg₃(PO₄)₂
Number of oxygen atoms = 3[2 x 4] = 24 oxygen atoms
For;
4Al₂O₃;
Number of oxygen atoms = 4 x 3 = 12 oxygen atoms
In 3Mg₃(PO₄)₂ , there are 24 - 12 = 12 more oxygen atoms than in 4Al₂O₃;
I would assume it would be impossible or unlikely