How Do You Solve Orbit Equations to Derive Kepler's Third Law?

[gunestheone]

Hi,
i need to solve the orbit equations that leads to Kepler's third law.
The equations are :

l = r * [e * cos(theta - theta0) - 1]

and

l = r * [e * cos(theta - theta0) + 1]
where l = (J * J) / (m * k)

 

[cristo]

What attempts have you made to answer this question?