Sum up with combination issue

[Myst]

Good afternoon,
Thanks in advance for your help, we ask for help to solve the exercise below,

I've been trying to solve it for hours, it's not clear to me what to do

 

[Steven G]

Yes, this is a great place to ask for help. Can you please tell us what type of help you need? Maybe tell us where you are stuck? After spending hours on this problem you must have some work to post. Please do so and then we can proceed from there.

 

[Myst]

I think I have solve it, I will post here maybe it will help someone in the future, and also if you can check it i will thank you a lot

 

[pka]

Good afternoon,
Thanks in advance for your help, we ask for help to solve the exercise below,
View attachment 20596
I've been trying to solve it for hours, it's not clear to me what to do

There is no question posted. What you posted is a statement, nothing more.
Unless we know what \(n=\) then the post is meaningless.
Are you asking for that sum to be expanded?