This one's a discrete mathematics question strictly due to its notation.

Compute the exact sums of the following:(SHOW YOUR WORK)

a) $\displaystyle \sum_{i=k}^n7$ where $\displaystyle k$ is an integer between 1 and $\displaystyle n$ (I have no idea how to answer this one)

b) $\displaystyle \sum_{k=1}^n(3k^2-2n+8)$ (I believe I have the answer, but I lack the steps and process to reach the answer)

c) $\displaystyle \sum_{j=1}^n\sum_{k=1}^j\sum_{i=k}^n(i-k)$ (I may have answered this one, but I'd like confirmation)

d) $\displaystyle \sum_{j=18}^{99}12$ (This one was easy, and I've already answered it)

This shouldn't be too hard for some people, but I'd rather not get these wrong.