To determine the pH of the HBr solution, we can use the relationship between the concentration of hydrogen ions (H+) and pH:

$$pH = -\log_{10}[H+]$$

Given that [H+] = 2.1 x 10-6 M, we can substitute this value into the equation:

$$pH = -\log_{10}(2.1 \times 10^{-6})$$

$$pH = 5.68$$

Therefore, the pH of the HBr solution is approximately 5.68.