(x+1)(x-2)+(x+2)(x-1)=

(x+1)(x-2)+(x+2)(x-1)=x²-2x+1x-2+x²-1x+2x-2=2x²-4

(x+1)(x-2)+(x+2)(x-1) = x² - 2x + x - 2 + x² - x + 2x - 2 = 2x² - 4 = 2 (x² -2)