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SmartStudent
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.
There are 4 mg
1 oxygen
1 hydrogen
Then you have to double that beach use of the 2
So the answer is 12