Well, it would be 1+3 which is 4. Then 4+5 which is 9. Then 9+7 which is 16 so your final answer would be 16
3 | 1 5 -8 6
. | 3 24 48
- - - - - - - - - - - - - - - -
. | 1 8 16 54
That is to say,

The remainder is 54.
Another way of doing it is to apply the polynomial remainder theorem, which says the remainder upon dividing a polynomial
by
is exactly
. So recognizing that the listed coefficients refer to

we find

The difference means subtract
89.4 - 13.621 = 75.779