Answer :
According to the definition, is a parabola is in the standard form of (x-h)^2=4p(y-k), then the directrix is y=k-p. In this case, the parabola function is x^2=8y, so h=0, p=8/4=2, k=0. Plug the values of h, p, k, into y=k-p, the directrix is y=-2.