Question 6644
This is a completing the square problem:

{{{x^2 + y^2 - 10x + 8y -40 = 0}}}


Get the x terms together, and the y terms together, and leave a space after each to complete the square.  Also add + 40 to each side of the equation:

{{{x^2 - 10x + ____ + y^2 + 8y + ____ = 40 + ____ + ____ }}}


For the first blank, take half of the 10, which is 5 and square to get 25.
For the second blank, take half of the 8, which is 4 and square to get 16.  Add these to both sides of the equation:
{{{x^2 - 10x + 25 + y^2 + 8y + 16 = 40 + 25 + 16 }}}
{{{ (x-5)^2 + (y+4)^2 = 81}}}


Therefore this is a circle whose center is at (5, -4) and {{{r^2 = 81}}} so {{{r=9}}}. 


R^2 at SCC