@KateS93 You are adding 6 hours and 30 minutes to 3 hour and 36 minutes. That's 10 hours and 6 minutes. But you probably want to calculate the duration between the two time stamps. An I assume the times are 6:30 am and 3:36 pm. Use =F10-E10 in stead and custom format the cell as [h]:mm. The square brackets are to allow a duration longer than 24 hours. Without them, a duration of e.g. 25 hours will be displayed as 1:00.
Edit: The latter is actually demonstrated on the next row. 12:30 + 13:00 = 25:30 which gets displayed as 1:30, i.e. 1 and a half hour past midnight.