6: Since there is no factorial and even no expression with k as a multiplicative factor, it should be possible to sum up this series using the geometric expansion.
There are two problems that need to be addressed. First, the terms are not of the type q^k. Get rid of the x and then rewrite the terms properly. Second, fix the beginning of indexing. This can be changed to 0 either using the general add-subtract trick or by factoring out the lowest power present.

Next hint
Answer