The equivalent weight of $\text{Mg}{\left(\text{OH}\right)}_2$  is

$\text{Mg}{\left(\text{OH}\right)}_2$  having two replaceable hydrogen ions, so its acidity is two and its molecular weight is 58.

$\therefore \text{Eq}\text{.wt}\text{of}\text{Mg}{\left(\text{OH}\right)}_{\text{2}}=\frac{58}{2}=29$