Transit police arrested a 20-year-old man on Tuesday on suspicion of two carjacking attempts in Quincy last week, officials said. Tyreke Mastin of Dorchester was arrested on at the Hynes Convention Center T station in Back Bay, Transit police said. Two women reported incidents of a man following them to their cars in the Quincy Adams parking garage on Thursday just before 8 p.m., prompting a community alert on Friday. In the first case, the man is accused of trying to enter the passenger side of the woman’s vehicle, shouting at her to open the door before she managed to drive away, Transit police said. Mastin is charged with attempting to commit a crime (carjacking).