the inner summation does not have an indexing variable in the summation and can be written as:

4*(sum(1, j = 1 .. i)) -> this means 4 times the summation of 1, with j=1 ending at i

this gives you 4*i

in your text you should have a proof of why that is.

You should also have some common identities that often come up with their proofs.

having a single indexing variable with nothing else happening to it, is one of those identites.

perhaps you can do this problem without the identity?