How can i can the graphing the function

in short, you simply pick a few random "x" values, to get the "y", and that's your point, for example say x = 2, then y = -(2)² - 4 => y = -8, that gives us the point of (2, -8), and so on.

we can start off by finding the vertex, the U-turn of the graph, and then just pick a point to its left side and a point to its right side, and we can get the vertex of that by

and since it's a vertical parabola, the axis of symmetry comes from the x-coordinate of the vertex, namely x = 0, check the picture below.