997 = 1000 - 3 998 = 1000 - 2 999 = 1000 - 1
997 x 998 - 999² = (1000 - 3) x (1000 - 2) - (1000 - 1 )²
= 1000x1000 - 2x1000 - 3x1000 -3x-2 - (1000² - 2x1000x1 + 1²)
= 1000 000 - 2000 - 3000 + 6 - (1000 000 - 2000 + 1)
= 1000 000 - 5000 + 6 - 1000 000 + 2000 - 1
= - 5000 + 2000 + 5
= - 3000 + 5
= - 2 995