Actually, the particle has a negative acceleration, and hence, you should do (0-60)/15 = -4 m/s^2
A particle is moving with a velocity of 60.0 m/s in positive x direction at t= 0 and t = 15.0 s the velcity decreases uniformly to zero . What was the acceleration during this 15.0-s interval ? what is the significance of the sign of your answer ?
my answer is
Vo = 0
a = 60 - 0 / 15 = 4