Ellipse

The ellipse $$\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}=1$$ and the straight line $$y=mx+c$$ intersect in real points only, if?
Given two fixed points $$\mathrm{A}$$and $$\mathrm{B}$$ and $$\mathrm{A}\mathrm{B}=6$$, then the simplest form of the equation to the locus of $$\mathrm{P}$$ such that $$\mathrm{P}\mathrm{A}+\mathrm{P}\mathrm{B}=8$$  is: