Question

how to graph y=-8x^2+4x+0

Answer 1

y = -8x^2 + 4x

You don't need the zero

But graph the vertex. Then graph the zeroes. Then graph two other points and draw a parabola through the points.

Answer 2

If you factor it you get
y = -4x(x^2 - 1)
y = -4x(2x + 1)

So Basically the zeroes are x = 0 and x = -1/2
That should get you started. If you know how to find the vertex, then it will help you.