I am currently studying Kinematic Chapter and I am facing problem in the following question: The initial velocity of a particle is 15ms-1 along the Ox direction(Ox is a straight line). After 5.0 s its velocity is 15 ms-1 at an angle of 60 degree to the Ox direction. Find the magnitude and direction of the average acceleration of particles. The answer given is 3.0 ms-2 at 120 degree to ox direction, but I don't know the steps. Please help me.