Answer
The formula of magnesium hydroxide is : $Mg(OH)_2$
Work Step by Step
** Since the last name is "hydroxide", we can assume that it will act as a base in water.
"Magnesium" is the full name for the element: $Mg$, which is from the group II of the periodic table. Therefore, its charge will be equal to $2+.$
"Hydroxide" represents the ion formed by a hydrogen and an oxygen atom, with a single negative charge: $OH^-$. (Hydroxide ion)
To balance the charge, the compound must have 2 $OH^-$, so the total charge is -2, and we can sum that with "+2" from the $Mg^{2+}$, to get 0.
$Mg(OH)_2$
