The vertex of the parabola is the highest or lowest point of the graph.
y=-4x^2+8x-12 = -4 (x^2 -2x +3)
Lets work with this now: x^2 -2x +3
x^2 -2x +3 -> what is the closeset perfect square?
x^2 -2x +1 = (x-1)^2
So
x^2 -2x +3 = (x-1)^2 +2
Replacing to original
y=-4x^2+8x-12 = -4 (x^2 -2x +3) = -4 ((x-1)^2 +2) = -4 (x-1)^2 - 8
The min or max point is where the squared part = 0
So when x=1 , y= -4*0-8=-8
This will be the max of the parabola as there is - for the highest factor (-4x^2)
The max: x=1, y= -8