422

$x^2+8x+16=\left(x+4\right)^2$
$y^2-10y+25=\left(x-5\right)^2$
$x^2+y^2+8x-10y-8=0$
$x^2+2x\cdot4+...+y^2-2y\cdot5+...=8$
$x^2+2x\cdot4+16+y^2-2y\cdot5+25=8+16+25$
$\left(x+4\right)^2+\left(y-5\right)^2=49$