Réponse:
J=(x+7)(3-2x) + (5x-2)(4x+1)
=3x-2x²+21-14x+20x²+5x-8x-2
=18x²-14x+19
K=(5x-7)(-2x+8)-(3x-5)(x-9)
=-10x²+40x+14x-56-(3x²-27x-5x+45)
=-10x²+40x+14x-56-3x²+27x+5x-45
=-13x²+36x-101
L=(2x+3)(6x-8)-3(9x-4)(-5x-1)
=12x²-16x+18x-24+(-27x+12)(-5x-1)
=12x²-16x+18x-24+135x²+27x-60x-12
=147x²-31x-36
