From a Foundations of mathematics course The main cable hang

From a Foundations of mathematics course:   


The main cable hanging from the tops of the suspension towers of the Golden Gate Bridge is shaped as a parabola; the vertex of this parabola is the lowest point of the cable, lying six feet above the level of the road. The towers are approximately 520 feet above the level of the road and stand 4200 feet apart. Use these facts to find an equation representing the shape of the cable, placing the cable’s vertex at the origin of your coordinate plane.

Solution