Ok so you have to use PEMDAS . so first you have to get rid of the parenthesis.
2 x 6x = 12x then 2 x -5= -10
12 + 5x -8 = 12x -10
combine now: 12 - 8 = 4
5x + 4 = 12x - 10
- 4 -4
5x = 12x -14
-12x=-12x
-7x = -14
-14/-7
x=2
Solve 5+8 and get 13(3+x) then muliply 13×3 and 13×X... you will end up with 39+13x
7/28 is the answer to your question.
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)