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)
Yes lil mosey is in fact white.
The answer is 6 shirts
12.5x+8=83
12.5x=83-8
12.5x=75
x=75/12.5
x=6
C. First person -hellobrains