Change in speed around elliptical orbit

[shnav34]

A comet is in an elliptical orbit around the Sun. Its closest approach to the Sun is a distance of 5e10 m (inside the orbit of Mercury), at which point its speed is 9e4 m/s. Its farthest distance from the Sun is far beyond the orbit of Pluto. What is its speed when it is 6e12 m from the Sun? (This is the approximate distance of Pluto from the Sun.)

how do I find the speed? (equations needed)

 

[mgb_phys]

Two options
At any point in the orbit the centrifugal force (depends on speed) must balance the gravitational force (depends on radius)

The energy is constant during the orbit, kinetic energy (depends on speed) + potential energy (depends on radius) must be the same

 

[shnav34]

i have a question, because its an ellipse how do you determine the radius, or do you mean the distance from the sun to the two points on the ellipse?