Unsure if it is valid to define a real number b less than abs(x-a) if x is a variable

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I am currently working through some questions from my course to do with proving continuity. In one question I have to go through and spot the error in a given proof. In one line there is defined a fixed real number $b$ where $0\lt b \lt \lvert x-a \rvert$ where $a$ is a given real number and $0 \lt \lvert x-a \rvert \lt \delta$ holds. I thought this might be an illegitimate definition, because $x$ is a variable and so its value isn't fixed and hence $b$'s value wouldn't be fixed either. Is this correct?