Salut ! :)
Factoriser -10 (x - 7)² + 90
-10(x - 7)² + 90 = 90 - 10(x - 7)²
= 9×10 - 10(x - 7)²
= 10 (9 - (x - 7)²)
= 10 (3² - (x - 7)²)
= 10 (3 - (x - 7))(3 + (x - 7))
= 10 (3 - x + 7)(3 + x - 7)
= 10 (-x + 10)(x - 4)
Voilà, j'espère que tu as compris ! :)