Answer:
Cu + 2 HBr----> CuBr₂+ H₂
Explanation
This is because in the unbalanced equation, you have: 1 Cu atom, 1 H atom, and 1 Br atom on the reactant side; on the product side, you have 1 Cu atom, 2 Br atoms, and 2 H atoms. Since the Cu atoms are balanced, you don't have to worry about balancing it, but for the H atoms and Br atoms, you have to balance them.
If you have other atoms, it's best to balance them first before you balance oxygens or hydrogens (if you have them in a reaction) because the oxygens and hydrogens tend to balance themselves out if you just focus on balancing other atoms, like in this equation for example.
That said, for the Br atoms, since CuBr2 has 2 Br atoms, and HBr has 1 Br Atom, add a coefficient of 2 to HBr so that you have 2 Br Atoms on both the product and reactant side, so that's balanced. The H also automatically balances itself out, as I said previously, because when you add the coefficient of 2 on the HBr, then there not only 2 Br Atoms but also 2 H atoms, and since there are 2 H atoms on the product side too, the H is balanced. Therefore, Cu + 2 HBr----> CuBr₂+ H₂ is the balanced equation.
