I don't know how to do that directly. When you enter x^2/25 + y^2/16 = 1, W|A tells you that this is the equation of an ellipse. If you then enter ellipse, W|A gives you parametric equations for the ellipse with Cartesian equation \(\displaystyle \frac{x^2}{a^2} + \frac{y^2}{b^2} = 1\).