Answer
The concentration of $HBr$ is equal to $5.6 \times 10^{-5}M$
Work Step by Step
Since $HBr$ is a strong acid, $[H_3O^+] = [HBr]$
$[H_3O^+] = 10^{-pH}$
$[H_3O^+] = 10^{- 4.25}$
$[H_3O^+] = 5.6 \times 10^{- 5}M$
Therefore, $[HBr] = 5.6 \times 10^{-5}M$
