Calculating a Point's X position on an Ellipse, given pos Y How can I calculate for the X coordinate given the Y value of the position? Which the Y position is 10 units, as seen in the image below.
We know the X diameter is 200 and the Y diameter is 150.


If you have an ellipse, you can use the standard equation for an ellipse:
$$\left( \frac{x}{a} \right)^2 + \left( \frac{y}{b} \right)^2 = 1,$$
where $a = \frac{200}{2}$ and $b = \frac{150}{2}$. Then you can find the $x$ value simply by substituting your $y = 10$ value and solving the above equation algebraically for $x$. 
Note that when you solve for $x$, you will have to take a square root, which means you should have a $\pm$ in your answer (since there are two x-values for which $y = 10$).
You can read more information about ellipses here:
https://en.wikipedia.org/wiki/Ellipse#Equations
