An ellipse and a hyperbola have their principal axes along the coordinate axes and have a common foci separated by a distance $2\sqrt{13}$,the difference of their focal semi axes is equal to $4$.If the ratio of their eccentricities is $\frac{3}{7}$.Find the equation of these curves.
The distance between the foci of both ellipse and hyperbola is $2\sqrt{13}$.
So $2c_1=$distance between the foci of ellipse$=2\sqrt{13}$
$2c_2=$distance between the foci of hyperbola$=2\sqrt{13}$
$\frac{\text{eccentricity of ellipse}}{\text{eccentricity of hyperbola}}=\frac{3}{7}$
I do not understand what is the meaning of "the difference of their focal semi axes" and how to proceed further.