The answer for number 7 is x=3
You can find the remainder right away by simply plugging in

. The polynomial remainder theorem guarantees that the value of

is the remainder upon dividing

by

, but I digress...
Synthetic division yields
3 | 2 -11 18 -15
. | 6 -15 9
- - - - - - - - - - - - - - - - -
. | 2 -5 3 -6
which translates to

(and note that

, as expected)
Try and get the variable by its self by -81 from both sides then divide it by 7 and get ur answer.
The answer is: 
The explanation is shown below:
1- You have the following expression given in the exercise:

2- When you multiply signs, you obtain:

3. Now, you must add like terms, as following:

4. Therefore, you have that the expression simplified is:

The statement “3, 9, 81, 243 is an example of an infinite
arithmetic series” is FALSE. I am hoping that this answer has satisfied your
query and it will be able to help you in your endeavor, and if you would like,
feel free to ask another question.