10b-21-b² bagaimana cara memfaktorkanya tolong jawab y!!

-b²+10b-21
(-b+7)(b-3)

10b - 21 - b² = 0
-b² + 10b - 21 = 0
_____________(-1)
b² - 10b + 21 = 0
Faktor
..... x ..... = 21
......+...... = -10
→ -7 dan -3

b² - 10b + 21 = 0
b² - 3b - 7b + 21 = 0
(b² - 3b) -(7b + 21) = 0
b (b - 3) - 7 (b - 3) = 0
→ (b - 7) (b -3)

Atau
-b² + 10b - 21
-b² + 3b + 7b - 21
-(b² + 3b) + (7b - 21)
-b (b - 3) + 7 (b - 3)
→ (-b + 7) (b - 3)