Read this and answer the question. If the question is unanswerable, say "unanswerable".

The Obama administration began to reengage in Iraq with a series of airstrikes aimed at ISIS beginning on 10 August 2014. On 9 September 2014 President Obama said that he had the authority he needed to take action to destroy the militant group known as the Islamic State of Iraq and the Levant, citing the 2001 Authorization for Use of Military Force Against Terrorists, and thus did not require additional approval from Congress. The following day on 10 September 2014 President Barack Obama made a televised speech about ISIL, which he stated "Our objective is clear: We will degrade, and ultimately destroy, ISIL through a comprehensive and sustained counter-terrorism strategy". Obama has authorized the deployment of additional U.S. Forces into Iraq, as well as authorizing direct military operations against ISIL within Syria. On the night of 21/22 September the United States, Saudi Arabia, Bahrain, the UAE, Jordan and Qatar started air attacks against ISIS in Syria.[citation needed]

What law did Congress cite, allowing it to take action against ISIS?
unanswerable