Hello goursiddhant

I hope you are doing good. I would like to know you that

accelaration is actually changing of velocity per sec

intitalc velocity =0 m/s (as start from rest)

final velocity=75m/s

change in velocity=75-0=75m/s

hence required acceleration will be

=75/15=5 m/(s^2)

If you have any other doubt then you can always reach us to clear them :)