vertexes lie on directives . Pt locus of 3rd vertex is $\frac {y^2}{b^2}-(k+l)^2=0$

Question

A triangle circumscribed a ellipse $\frac {x^2}{a^2}+y^2+a^2=1$ and two vertexes lie on directives . Pt locus of 3rd vertex is $\frac {y^2}{b^2}-(k+l)^2=0$