Hi synkk
The wording is probably confusing you.
First, did you do (a) ? do it if you didn't
Then, as micromass told you, Un is just a number, well, in this case, you are dealing with a sequence of numbers, U1, ... Un
You have a generic formula for their sum, which is a function of n, being the result of summing the n elements U1,...Un
so, if you subtract the sum of all the elements, U1,...Un-1, what are you left with ?
Hope this helps so that you continue alone
Cheers...