When we sort the terms in descending order, we get 3x^6 - 4x^3 + 8x + 9
The exponents decrease (from 6 to 3 to 1 to 0) with the largest exponent term on the left side and the smallest on the right side.
Fill in the missing terms to get 3x^6 + 0x^5 + 0x^4 + (-4x^3) + 0x^2 + 8x^1 + 9x^0
So the sequence of coefficients are 3, 0, 0, -4, 0, 8, 9