Here are some ideas. Pick a point on your curve (x,y(x)). You can find the slope of the line that passes through the origin and your point. You also know that the slope of the horizontal line is zero. At the point you picked on your curve you can find the slope of the tangent line. There is an angle between your first curve and your tangent line (angle of incidence) and an angle between your tangent line and your horizontal line (angle of reflection) and these are equal. By relating your angles to your slopes you'll find a differential eqution for y(x).