This study is being done to answer the following question: Among patients with early-stage TNBC who achieve a pCR after preoperative chemotherapy in combination with pembrolizumab, will observation after surgery be as effective as completing 6 more months of pembrolizumab after surgery at preventing cancer from coming back? We are doing this study because we want to find out if observation is as good as the usual approach for your breast cancer. The usual approach is defined as care most people get for breast cancer