How do I find the directrix of the parabola y=2x^2?

To find the directrix of a parabola wohse equation is ##y=ax^2+bx+c##
you can use this formula:
##y= (-1-(b^2-4ac))/(4a)##
So in case of this parbola ##y=2x^2##
the directrix is
##y=(-1-(0^2-4*2*0))/(4*2)##
##y=-(1)/(8)##