Can you explain what tax research has to do with the answer like how is the answer supported by internal revenue code?

On March 10, 2019, Elizabeth, a college professor, purchased a house for $300,000. She did not move into the house until August 8, 2019. On August 1, 2020, she accepted a position as a visiting professor at Hogwatts University for one year and moved to Liverpool where she rented an apartment. While away at Hogwatts, two of her former students lived in the house but did not pay rent. She returned to the house on August 1, 2021, and lived in the house until July 15,2022 , when she sold the house for $500,000. The realtor's commission was $35,000. Determine her recognized gain. A partial list of research sources is: - Reg. Sec. 1.121-1(c)