Tolong bantu ya soal mat. Thanks.

Titik puncak (xp,yp) = (4,2) dan melalui (x,y) = (2,0)
y = a(x - xp)^2 + yp
0 = a(2 - 4)^2 + 2
0 - 2 = a(-2)^2
-2 = 4a
a = -2/4 = -1/2
y = a(x - xp)^2 + yp
y = -1/2 (x - 4)^2 + 2
y = -1/2 (x^2 - 8x + 16) + 2
y = -1/2 x^2 + 4x - 8 + 2
y = -1/2 x^2 + 4x - 6
a = -1/2, b = 4, c = -6
abc = -1/2 . 4 . -6 = 12