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)##