The equilibrium system between hydrogen gas, bromine gas, and hydrogen bromide gas is given. H2(g) + Br2 (g) <----> 2HBr(g).?

Bromine gas exists as Br₂ molecules, but not Br atoms. Hence, it is Br₂(g) instead of Br gas.

Hence, the equation should be :
H₂(g) + Br₂(g) ⇌ 2HBr(g)
OR: 2HBr(g) ⇌ H₂(g) + Br₂(g)

Both of the above equations are correct. However, someone only accepts the first equation which follows the order of giving components, i.e. hydrogen gas, bromine gas and then hydrogen bromide gas.