Answer
$$2HBr(aq) + NiS(s) \longrightarrow H_2S(g) + NiBr_2(aq)$$
Work Step by Step
1. This reaction has a sulfide and an acid as its reaction. Thus, it will produce $H_2S(g)$ and a salt made of the other ions:
$$HBr(aq) + NiS(s) \longrightarrow H_2S(g) + Salt$$
The remaining ions are: $Ni^{2+}$ and $Br^-$, therefore, the salt is: $NiBr_2(aq)$
$$HBr(aq) + NiS(s) \longrightarrow H_2S(g) + NiBr_2(aq)$$
2. Now, balance the equation.
The number of $H$s and $Br$s on the products side is equal to 2, thus, we just need to put a $2$ before $HBr(aq)$:
$$2HBr(aq) + NiS(s) \longrightarrow H_2S(g) + NiBr_2(aq)$$
