Write a balanced chemical equation for the reaction of magnesium with hydrochloric acid (hci (aq) to form magnesium chloride and hydrogen gas:

Mg (s)       +    HCl (aq)    →  MgCl₂(s)   + H₂(g)

Looking at the equation :

We have 1 Mg at the left hand side and 1 Mg as well on the right hand side.
So that is balanced. 

We have 1 H at the left hand side and 2 H on the right hand side.
So that is not balanced.  Same for Chlorine. Cl.

We add 2 to the HCl on the left hand side and that balances it.

Mg(s)       +    2HCl(aq)    →  MgCl₂(s)  + H₂(g)

The equation is
Mg + 2HCl -> MgCl2 +H2