Change of Duties

I was hired by a company for a “lower” salary than my previous job.

HR told me that the job would not be as involved as my previous employment, so the “larger” salary could not be warranted. Approximately three weeks later, I was given the sametype of work I did at my previous place of employment and for which a larger salary was unwarranted. How do I tactfully bring that up to my supervisor and HR.