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[SOLVED] Ellipse - Focus and Directrix

An ellipse can be determined by means of a focus and a directrix. In the diagram, the directrix of an ellipse is the line $\displaystyle x = \frac {25}{3}$ and a focus is $\displaystyle (3,0)$. The relationship determining the ellipse is $\displaystyle PF = \frac {3}{5}PD$ for all point P.)

a. Use the distance formula to write and expression for $\displaystyle PF$, and set it equal to $\displaystyle \frac {3}{5}PD$. (Hint: You should not need the distance formula for $\displaystyle PD$.

b.Simply the equation you wrote in part (a), and put it in the graphing form of an equation of an ellipse. What is the length of the minor axis of this ellipse.

I posted the picture of the diagram below made in paint as best as it resembles on the paper. Ok...so my question is how do you solve this? I know the equation for and ellipse is $\displaystyle \frac {(x-h)^2 }{a^2} + \frac {(y-k)^2}{b^2} = 1$. I have know idea though what a directrix is as I haven't learned it yet.