Well, since the race is a total of 5000m, and each lap is 100m, you would have to divide these two numbers to see how many laps around the pool it would be:
5000/100 = 50
Since his steady pace for a single lap is 68 seconds, you would multiple the number of laps, by the amount of time it would take him:
68*50 = 3,400 seconds
So in total it would take him 3,400 seconds, or 56 minutes 40 seconds.
