Question 412786
The standard form to use is (x-h)^2=4p(y-k), with (h,k) being the (x,y) coordinates of the vertex, and p the distance from vertex to focus and vertex to directrix in the other direction on the axis of symmetry. For the given equation, x^2=16y,it is a parabola that opens upwards,the vertex is at (0,0),axis of symmetry is x=0, and 4p=16 or p=4.

The directrix, therefore, is y=-4
see the graph below:

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{{{ graph( 300, 300, -10, 10, -10, 10, x^2/16,-4) }}}