That depends on the average velocity over the 50 metres.
average acceleration = change in velocity / time
time = distance / average velocity
change in velocity is 5 m/s
distance is 50 m
So you need the average velocity to solve. Or know something about the acceleration curve.
For example if the motorcycle stopped in the first 5 metres waited for an hour then accelerated up to 20 m/s by 50 m the average acceleration would be different then if it uniformly accelerated.