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We need a stoichiometric equation that represents the reaction between $Mg(OH)_2$ and $HCl$, and here it is............

$Mg(OH)_2(s) + 2HCl(aq) rarr MgCl_2(aq) + 2H_2O(l)$

Here, $1$ $"equiv"$ of magnesium hydroxide reacts with $2$ $"equiv"$ hydrochloric acid. In the reaction that was performed, there were $3.0*mol$ hydrochloric acid, and stoichiometric equivalence a $1/2$ $"equiv"$ of $"magnesium hydroxide"$, i.e. $3/2*mol$.