What have tried? You really have to be trying something before you come here asking for help. If you have done work, that is great! Then you need to post your work.
Have you written out some terms on the left hand side? Maybe you can somehow end up with multiplying nC1 with nC1 and nC2 with nC2 etc and then the results will follow. Maybe you can do something that will separate (nCk)^2 into something helpful.
BTW, what I wrote above in my 1st paragraph is in the forum rules. I would really appreciate it if you follow those rules. Thank you.
This is a very natural, intuitive presentation of it for my way of thinking, although it seems more like a logical projection then a combinatorical proof...perhaps it is valid enough, binomial presentation seems a bit hardcore to think of it in an exam.
If you put equal signs then maybe I could follow your work better. It is hard to see if two line are equal or they are just one expression that took two lines to write. I will look again at your work now I now there are in fact three dots.
The third to the last line ends, that is no three dots. You noticed that each term is being squared by writing that in the next line. But what is that line not ending at (nCn)^2? Why does it continue for ever and most importantly why does it equal that last line?