equation of a circle is (x-x1)+(y-y1)=r²
when x1 and y1 are the coordinates of the centre
So you can substitute the centre in to get (x-5)²+(y+9)²=r²
When you draw a diagram it's obvious the radius is 9
9²=81
Therefore equation is (x-5)²+(y+9)²=81
Any questions, just ask :)
