Some basic definition you should know follow:
        Displacement:  How far away something is from its orginal position
        Average Velocity:  The average distance over the average time
        Instantaneous Velocity:  The velocity at which something is traveling at a point
                                             in time.
        Acceleration:  The rate of change of your velocity

    Basic equations you should know:
        V=d/t
        average V=(Vi+Vf)/2
        a=v/t
        average a= (Vf-Vi)/t
        D=Vit+.5at^2
        (Vf)^2=(Vi)^2+2ad
 

Start this problem by first understanding what it is asking.  We want to find the acceleration and in order to do this we must first find the equation we are going to use.

You know the Vi and the Vf and the distance,  Now you just plug your known variables in and solve.  You should get