We know the equation of motion v = u+ at, where v is the final velocity, u is the initial velocity, a is the acceleration and t is the time taken.
In this case Final velocity = 4.5 m/s
Time taken = 7 seconds
Initial velocity = 0 m/s
[tex]v=u+at\\ \\ 4.5 = 0+a*7\\ \\ a = 0.643m/s^2[/tex]
Car's average acceleration while it rolled down the hill = 0.643m/[tex]s^2[/tex]