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pass the parameters as a refrence.
void unitconversion (double &lengthme, double &lengthcent);
remove the return statement in unitconversion
don't cout<<unitoutput(lengthme, lengthcent);
...

Errors:
1. You haven't called the unitconversion function.
2. A function can return only one value at time, you might pass the parameters as a reference.
3. don't cout<<unitoutput(lengthme,...