First, it helps to know what comprises a tragic hero. The classic tragic hero has some type of tragic character flaw which creates an inner struggle, leads to his making a serious error in judgment, and leads to his eventual downfall and death. So, you would then apply those factors to Marcus Brutus to show that he fits the definition. Something like "Marcus Brutus' tragic flaw, [define it], which results in his ultimate demise, paints him as the classic tragic hero."
I hope this helps!