The bike is maintaining "constant velocity". He's moving at 15 m/s when we see him for the first time, 15 m/s later that day, and 15 m/s next week.
The car starts from zero, and goes 4.0 m/s FASTER each second. After one second, it's going 4.0 m/s. After 2 seconds, it's going 8 m/s. And after 3 seconds, it's going 12 m/s.
This is the point at which the question wants us to compare them ... 3 seconds. The bike is moving at 15 m/s and the car has sped up to 12 m/s. The bike is moving faster than the car.
If we hung around and kept watching for another second, the car would then be moving at 16 m/s, and would be moving faster than the bike. But we lost interest after answering the question, and we left at 3 seconds.
