Répondre :
a) x² - 5x + (2x² + 7x - 8)
=x²-5x+2x²+7x-8
=3x²+2x-8
b) (3x² - 5x - 4) - (-4x² + 7x + 5)
=3x²-5x-4+4x²-7x-5
=7x²-12x-9
c) -(6x² - 3) - (3x² + 2x - 3)
=-6x²+3-3x²-2x+3
=-9x²-2x+6
=x²-5x+2x²+7x-8
=3x²+2x-8
b) (3x² - 5x - 4) - (-4x² + 7x + 5)
=3x²-5x-4+4x²-7x-5
=7x²-12x-9
c) -(6x² - 3) - (3x² + 2x - 3)
=-6x²+3-3x²-2x+3
=-9x²-2x+6