Note the equation v = v0 + at, where v is the final velocity in meters per second, v0 is the initial velocity in meters per second, a is the acceleration, and t is the time in seconds.
Fist convert the velocities so 30km/hr=8.333m/s and 15km/hr=4.167m/s
Plugging this into the equation, we get
4.167 = 8.333 + a(4)
=> -4.167 = a(4)
=> a = -1.042 m/s^2
