I've attached the question using the link since it would be too hard to format myself.
Essentially, What i've done so far is said;
From start to wall 1, the time is $\frac{d}{u}$
From Wall 1 to Wall 2, the time is $\frac{9}{2u}$
From Wall 2 to Start, the time is $\frac{3-d}{u}$
But when i add them together i do not get the desired result, im not quite sure where ive gone wrong
You used the original velocity in the third segment where you should have used $\frac23\cdot\frac23\cdot u$.