An inclination of 90 degrees indicates a polar orbit, in which the spacecraft passes over the north and south poles of the planet. As stated before, the numeric values for these keplerian elements can be determined by three radiometric observations. Solving Kepler's Equation enable one to obtain the orbital elements and there are various means of doing so such as Newton's method as well as other itterative solutions. To learn more about actually solving the equation one can go to various web pages on the matter or look in Orbits for Amateurs with a Microcomputer by D. Tattersfield. Kepler stated that a planets radius will sweep out a constant area in a given time no matter what part of the orbit it is in.
True anomaly is found in terms of mean anomaly.
Apparently, the only way to relate these True Anomaly and Mean Anomaly is through a third term which is Eccentric Anomaly. Then draw a line straight down from P so that it intersects with the horizontal diameter of the circle perpendicularly. Using an iterative method is generally used to solve these two equations but there are other ways to do so. Another method of finding the orbital elements is to use one of several programs that have been written for just that. I logged my progress in figuring out how to use this program here .