The number of corresponding to each coefficient required to write the balanced equation for the reaction is as below
2 HNO3 + Mg(OH)2 = 2H2O + Mg(NO3)2
the coefficient is therefore
a=2
b=1
c=2
d=1
2 moles of HNO3 reacted with 1 mole of Mg(OH)2 to form 2 moles of H2O and 1 mole of Mg(NO3)2