$\begin{aligned}
& \mathrm{pH}=10 \
& \mathrm{pOH}=4 \
& {\left[\mathrm{OH}^{-}\right]=10^{-4}}
\end{aligned}$
no. of moles of OH−=10−4
no. of moles of Mg(OH)2=210−4=5×10−5
$\text { mass of } \begin{aligned}
\mathrm{Mg}(\mathrm{OH})_2 & =5 \times 10^{-5} \times 58 \times 10^3 \mathrm{mg} \
& =2.9
\end{aligned}$