Additional Study Information: The purpose of this study is to test the safety of drugs called tazemetostat and belinostat when administered together. This study tests different doses of the drugs to see which doses are safer for people. There will be between 21 and 48 people taking part in this study. There are two parts in this study, a dose escalation (dose finding) part and a dose expansion part (where the established dose is tested further). In the dose escalation part of this study, different people will get different doses of the study drugs tazemetostat and belinostat. The first 3 people taking part in this study will get the lowest dose. If the drugs do not cause serious side effects, the next group of people in the study will get a higher dose. The study doctor will watch each group carefully as they increase the dose. The doses will continue to increase for every new group until people have serious side effects that require the dose to be lower. Once this dose is found, the dose escalation is stopped. In the dose expansion part of this study, the highest doses of the study drugs with manageable side effects will be given to 12 more people. This will help study doctors better understand the side effects that may happen with this drug combination.