u/u^2+4

What you wrote is this:


\frac u {u^2} +4


But I wonder if what you meant is this:


\frac u {u^2+4}


Please verify which is correct.

The domain is the allowable range of values for the independent variable, in this case u. So - as you consider this function, are there any values of u that would not be allowed? One thing to keep in mind with this type of problem is that you never want to divide by zero, so check to see if there are any values of u that cause the denominator to be zero.

Post back with what you get for an answer.