Did you try calculating it for a couple of different values of n? When n= 1, the sum is . The right side you want to show that equal to is .

Youcan'tprove that, it isn't true.

(Perhaps I am confused as to what "nCr" means. I thought that was the binomial coefficient but there is no "r" on the right, and it isn't an index so I assumed you meant "nCk".)