Agencies can only create rules and regulations if a law has been passed enabling them to do so - hence "enabling legislation." When an Agency is created, it is given a set of powers and rights to make such rules and regulations. The collection of agencies' rules and regulation come in two forms or stages, The Federal Register (collection of rules, etc. published daily) and The Code of Federal Regulations (the collection of permanent rules).
You will need to find the rule under debate so that you can see what power has been granted to the Agency(s) in order to determine if what is being disputed in the court case is within the Agency's right or power.
Additionally you may have to go back to the statute section of the guide and repeat some steps to find the act that originally created the agency as some Agencies have morphed over time. In the Statute that created the Agency, you should also find a list of rights and powers awarded to the Agency.
Be sure to cover the following...
1. Identify the Agency(s) involved in the Supreme Court case.
2. Discuss the legislation or order that created the Agency(s) and the range of powers and responsibilities it was given in the enabling act that created it.
3. What ruling or other action taken by the Agency(s) gave rise to the case brought before the Supreme Court.
For an explanation of what the Federal Register (FR) and the Code of Federal Regulations (CFR) are, the University of Minnesota has a fantastic guide, with brief and clear definitions.