What you must do in this case is write the kinematic equations to find the length of the plane.
We have then:
a = 2 m / s ^ 2
v = a * t + vo
r = a * (t ^ 2/2) + vo * t + ro
r = (2) * ((3 ^ 2) / (2)) + (0) * (3) + (0) = 9 m
answer:
the length of the plane is 9m
