Titik puncak (2,-3) dan melalui (-1,-2)

y - yp = a (x - xp)²
-2 - (-3) = a (-1 - 2)²
1 = 9a
a = 1/9

y + 3 = 1/9 (x - 2)²
y + 3 = 1/9 (x² - 4x + 4)
y = 1/9 (x² - 4x + 4 - 27)
y = 1/9 (x² - 4x - 23)