How many atoms are in 4Mg (OH) 2

There are 4 mg

1 oxygen

1 hydrogen

Then you have to double that beach use of the 2

So the answer is 12

The 2, written as a subscript, counts for only the hydrogen. 3 H2O has 6 atoms of H and 3 atoms of O. The 3, written as a coefficient, means that there are 3 "H2O"s.

Mg (OH) 2 has 1 atom of Mg, 2 atoms of O, and 2 atoms of H.