ka bantu ya please buat besok ujian mi

Ralat : Grafik melewati titik (0,-4) dan titik puncak di (2,-8)

Cari nilai a dulu :
y = a(x-xp)^2 + yp
-4 = a(0-2)^2 + (-8)
-4 = 4a - 8
4a = 4a = 1

Masukin nilai a = 3, x dan y nya gk disubstitusi lagi :
y = a(x-xp)^2 + yp
y = 1(x-2)^2 + (-8)
y = (x^2-4x+4) - 8
y = x^2-4x -4

titik potong sumbu y , c = -4
titik puncak
x = -b/2a = 2, 4a = -b
y = -D/4a = -8,
b²-4ac/4a = 8,
b²-4b/-b = 8,
b-4 = -8
b = -4
4/2a = 2
a = 1
f(x) = x²-4x-4