You should aim to do these calculations in your head or on a scrap of paper (i.e. without a calculator). The following logic may be helpful:
Enzyme units are expressed as µmol substrate converted per min.
If the question gives enzyme activity in nmol per min, divide by 1000 to convert to µmol.
Then multiply by the volume to get the total number of units.
To summarize, the total number of enzyme units is:
volume x substrate conversion rate/(1000 if nmol, or 1 if µmol)
Always do a ‘sanity check’ on your answer. Is it the right order of magnitude? E.g. with volumes of a couple of ml, for a rate of hundreds of nmol per min the final answer is likely to be in the range 0.1–0.5 enzyme units. If the rate is x µmol per min, then the answer will be of the order of magnitude of x.