persamaan fungsi kuadrat yang memotong sumbu X di titik (3,0) dan (-5,0), serta memotong sumbu Y di titik (0,-15) adalah ? mohon bantuan nya

(3,0) = (x1,0)
(-5,0) = (x2,0)
(0,-15) = (x,y)
y = a(x - x1)(x - x2)
-15 = a(0 - 3)(0 - (-5))
-15 = a(-3)(5)
-15 = a(-15)
a = 1

y = 1(x - 3)(x - (-5))
y = (x - 3)(x + 5)
y = x² - 3x + 5x - 15
y = x² + 2x - 15

y = ax²+bx+c

(3,0)
0 = 9a+3b+c

(-5,0)
0 = 25a-5b+c

9a+3b+c = 25a-5b+c
16a-8b = 0
2a-b = 0
b = 2a

(0,-15)
-15 = c

9a+3b+c = 0
9a+3(2a)-15 = 0
9a+6a = 15
a = 1

b = 2a = 2

y = ax²+bx+c
y = x² +2x -15