Use the polynomial remainder theorem. If

is a polynomial of degree

, then we can divide by a linear term

to get a quotient

and remainder

of the form

Then when

, we get

. In other words, the value of

at

tells you the value of the remainder upon dividing

by

.
So given that

, and the remainder upon dividing

by

is -8, we know that

, so


Since

is a polynomial (not a rational expression), then we know that

divides

exactly. In particular, the remainder term of this quotient is 0. We can use long or synthetic division to determine

. I prefer typing out the work for synthetic division:
-1 | 1 -4 15 k + 8
. | -1 5 -20
- - - - - - - - - - - - - - - - - -
. | 1 -5 20 k - 12
The remainder here has to be 0, so

.
Finally, we can get the remainder upon dividing

by

by evaluating

, which gives

.