Getting wrong answer in an (angular) impulse momentum problem

[Divya]

Homework Statement

Three masses m, 2m and 3m are connected by two massless and rigid rods
of length l which are currently perpendicular to each other, as shown in the
figure. If the masses initially travel at velocity u towards a vertical wall and
mass m undergoes a collision with the wall, determine the impulse delivered
by the wall to mass m if the final horizontal velocity of mass m is zero. There
is no friction between the wall and mass m. Assume that the tensions in the
rods are strictly longitudinal (because they are massless).

Relevant Equations

impulse = change in momentum
angular impulse = change in angular momentum

I have tried this same approach three times and I got the same answer. I can't figure out what's wrong. Btw answer is 12mu/(3+cos²α)
And yes, sorry for my shitty handwriting. If you can't understand the reasoning behind any step then please let me know.

 

[haruspex]

sorry for my shitty handwriting

Forum rules state that images are for diagrams and textbook extracts. Please type in your working, preferably in LaTeX.
Also, it helps if you provide some explanation of your approach and define all variables.

It looks to me as though you are taking the rod arrangement as rigid. It only says initially a right angle. I would take it as having a free joint.

In my experience, it rarely gains anything to bother finding the mass centre of such an assemblage anyway.

 

[Divya]

I am really sorry for not seeing the rules of forum before posting ( I am new here). Also thanks for your help. The 'free joint' interpretation gave the right answer.

 

[haruspex]

I am really sorry for not seeing the rules of forum before posting ( I am new here). Also thanks for your help. The 'free joint' interpretation gave the right answer.

well done