N(p+) = 12, N(n0) = 14, N(e-) = 10

Answer is: -601,2 kJ/mol
Chemical reaction: Mg(OH)₂ → MgO + H₂O.
ΔHrxn = 37,5 kJ/mol.
ΔHf(Mg(OH)₂) = −924,5 kJ/mol.
ΔHf(H₂O) = −285,8 kJ/mol.
ΔHrxn -enthalpy of reaction.
ΔHf - enthalpy of formation.
ΔHrxn=∑productsΔHf−∑reactantsΔHf. 
ΔHf(MgO) = -924,5 kJ/mol - (-285,8 kJ/mol) + 37,5 kj/mol.
ΔHf(MgO) = -601,2 kJ/mol.