Write a function to find the directrix of a parabola.
def parabola_directrix(a, b, c): 
  directrix=((int)(c - ((b * b) + 1) * 4 * a ))
  return directrix
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a function to find the directrix of a parabola.
def parabola_directrix(a, b, c): 
  directrix=((int)(c - ((b * b) + 1) * 4 * a ))
  return directrix[/INST]assert parabola_directrix(5,3,2)==-198
assert parabola_directrix(9,8,4)==-2336
assert parabola_directrix(2,4,6)==-130</s>