Coordinates

We can rewrite this as: $x^2-10x+y^2+16y=0$

We can now complete the square, and add the remainder to the right-hand side: $(x-5)^2+(y+8)^2=64+25$

In a circle's equation, the right-hand side is equal to $r^2$, where r is the radius. 

This means $89 = r^2$

Can you take it from here?