The chemical equation of magnesium and nitric acid with balancing process

Mg + HNO3 - > Mg (NO3) 2 + H2

On reactants side:

Mg=1

H=1

N=1

O=3

On products side:

Mg=1

H=2

N=2

O=6 (3*2)

Therefore to balance the equation, we must add 2 to HNO3 on the reactants side ...

Mg + 2HNO3 - > Mg (NO3) 2 + H2

Reactants:

Mg=1

H=2

N=2

O=6

Therefore, it's balanced.