input: Write a python function to determine the focal point of a parabolic curve.
  assert parabola_focal_point(5, 3, 2) == (-0.3, 1.6)
  assert parabola_focal_point(9, 8, 4) == (-0.4444444444444444, 2.25)
  assert parabola_focal_point(2, 4, 6) == (-1.0, 4.125)
output: def parabola_focal_point(a, b, c):
      x_vertex = -b / (2 * a)
      y_vertex = (4 * a * c - b**2) / (4 * a)
      focal_point_x = x_vertex
      focal_point_y = y_vertex + 1 / (4 * a)
      return focal_point_x, focal_point_y