find the difference

(6x^2-3x-9)-(2x^2+8x+3)

(6x^2-3x-9)-(2x^2+8x+3)

Remove parentheses and change the sign of the terms inside the parenthese

Combine like terms
6x^2 - 3x - 9 - 2x^2 -8x - 3

I leave the rest to you