Question

How do you figure out this equation?

Answer 1

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 :)