Question

Balance _Mg + _HCL = _MgCl2 + H2

Answer 1

_Mg + _HCL = _MgCl2 + H2
Separate the terms on each side: 

_Mg + _HCl = _MgCl2 + H2

Mg- 1                 Mg-1
               
H-1                    H-2

Cl-1                    Cl-2

Mg is balanced on both sides so move on to the next (put a 1 in the space). 

1Mg

There are two H's and two Cl's on the results side, so to balance the equation put a 2 as a coefficient for HCl and it'll all balance out.

2HCl

Balamced equation will be: 

 1Mg + 2HCL = 1MgCl2 + H2

Answer 2

1 Mg + 2 HCl ---->  1 MgCl2 + 1 H2