input: Write a function to find the focus of a parabola.
  assert parabola_focus(5,3,2)==(-0.3, 1.6)
  assert parabola_focus(9,8,4)==(-0.4444444444444444, 2.25)
  assert parabola_focus(2,4,6)==(-1.0, 4.125)
  
output: def parabola_focus(a, b, c):
      x_focus = -b / (2 * a)
      y_focus = (4 * a * c - b * b) / (4 * a)
      return (x_focus, y_focus)