This is a study involving an investigational (experimental) drug called MRTX849, being developed by Mirati Therapeutics, Inc., which is the company that provides MRTX849 study drug and is funding and organizing the study. Investigational means that MRTX849 has not been proven to help people with colorectal cancer and is not approved by regulatory authorities such as the United States Food and Drug Administration (US FDA). This study will evaluate the effectiveness of the combination of MRTX849 with cetuximab in subjects with colorectal cancer who are experiencing progression on or after first-line chemotherapy.