y = 2x^2-12x+16 = 2(x-3)^2 -2
y+2 = 2(x-3)^2
绕顶点旋转180°,顶点不变,2(x-3)^2前面变号：
y+2 = -2(x-3)^2
y= -2(x-3)^2 -2 = -2x^2+12x-20