Perform completing the squares for x and y separately to transform the given equation to the standard equation of the circle:


x^2 + y^2 + 8x - 10y = -5


(x^2 + 8x) + (y^2 - 10y) = -5                        <<<---=== first step - grouping


(x^2 + 8x + 16) + (y^2 - 10y + 25) = -5 + 16 + 25    <<<---=== identical transformations


 +  = 36.                                 <<<---=== complete the squares


You got the standard equation of the circle.


Answer.  The radius of the circle is   = 6.