a particle is moving along a straight line with constant acceleration. in an interval of T seconds it moves D metres; in the next interval of 3T seconds it moves 9D metres. how far does it move in a further interval of T seconds?

so I calculated the acceleration pretty simply, the velocity increased by 2D/T over 3T, so the constant acceleration is 2/3DT^-2. using this I calculated the distance travelled over the next interval of T seconds as being 0.5(3 + 3 + 2/3) = 20/6 D metres

the book says 5D metres. where did i go wrong?