thank guys! Now I understand how to solve the problem.
1)Mg2+(aq) + 2HCl(aq) -> MgCl2(aq) + 2H+(aq)
this reaction is incorrect, the net ionic equation cancels out completely, no net reaction occurs
2)g2+(aq) + 2H2O(l) <=> Mg(HO)2(s) + 2H+(aq)
This reaction is what you should be using.
3)Mg(HO)2(s) <=> Mg2+(aq) + 2HO-(aq)
This one is superflous since we it is indicated in the second equation that equilibrium in the formation of the solid has been achieved.
Thus the net neutralization reaction is
2H30+ + 20H- --->4H20(l)
=H30+ + 0H- ---->2H20(l)
the hydronium is from equation 2.
all of this, assuming that your equations were balanced properly