To complete task in one roll you have to get "5" or "6", so there are 2 possible outcomes that are correct, but there are total of 6 possible outcomes. So probability have to be some combination of these numbers, you can roll "5", so one number from six and also roll "6", so again one number from six. In total you can roll 2/6 numbers to be successful.
In two rolls you can get 6*6 possible outcomes, how many times you can be succesful? just write it down and you will see the answer. Also for 3 and 4 rolls. Maybe start with 5 rolls and you will see it imediately.