This is an odd duck, indeed !!!!
It is a rotated ellipse
In the form
√ [ ( x - a)^2 + ( y - b)^2 ] + √ [ (x - a ' )^2 + ( y - b ' )^2 ] = C
The foci are located at (a, b) and ( a ' , b ' )
[ Notice how this is very similar to finding the center of any conic....!!! ]
So....the foci here are ( 3, -4) and (-5, 8)
So....the distance between these = √ [ ( -5 - 3)^2 + (-4 - 8)^2 ] = √ [ 64 + 144] = √ [ 208 ] = 4 √13 units
