Given an ellipse centered at $(3,-3)$, and has a focus at $(3,-8)$.
What is the perimeter of a triangle that entirely lies within the ellipse and has two of its vertices on the foci of the ellipse and the other vertex lies on any point on the circumference of the ellipse?
The perimeter is the sum of the length of the principle axis of the ellipse and the distance between the foci. This is a definition of an ellipse. See this link for details.