Normal Distribution Basic Question

For some reason I have no idea how to do Normal Distribution anymore, and couldn't find much on it in my text book...

So if someone could kindly show me how to do this question in detail I'd be very grateful =).

Research has shown that the times taken for walkers to complete a track through a particular national park are normally distributed with a mean of 3.6 hours and a standard deviation of 0.5 hours.

Calculate the probability that a randomly seleted walker will take less than 3 hours to complete the track.