Check the Show Equation box and Window Setting [-40,40] x [-40,40). Select the Parabola-y conic; select the h, k sliders and use the sliders to graph the conic y = 40(x - 15). Find the equation of the directrix of this parabola with vertex (15,0) and focus (25,0). Use the interactive figure to find your answer. Use the left and right arrow keys to move along a slider as needed. Click here to launch the interactive figure. A horizontal parabola with vertex (15,0) and focus (25,0) will have a directrix equation