Answer:
4x^2 -16
Explanation:
As with any "what must be added" problem, you subtract the parts you have from the result you want:
(3x^2 +4x +8) + (2x^2 -6x +3) + what = 9x^2 -2x -5
Identify the stuff on the left that is not "what" and subtract it from the equation:
what = (9x^2 -2x -5) - (3x^2 +4x +8) - (2x^2 -6x +3)
what = x^2(9 -3 -2) +x(-2 -4 +6) +(-5 -8 -3)
what = 4x^2 -16 . . . . . . . must be added to give the desired sum