I would say E.......!!
v^3-k^3 = (v-k)*(v^2+vk+k^2)
Since vk>0 and we do not know an exact value, we cannot determine..
i would say E also, but maybe not
if vk>0, then v and k must both be - or both be +, but since you don't know which is bigger
if v-k=6, still doesn't give us help on the second part of the equation about (v*2+vk+k*2)
if you use both to substitute, etc, you can factor down to (2)(k*2 + 6K + 6), but then are stuck with k*2 + 6K + 6, which if you can solve, then you might have an answer for k, which you could plug back into v-k=6, and eventually get v*3-k*3