(x- h)^2+(y- k)^2= r^2
In this form, ( h, k) is the center of the circle and r is its radius.
We are told that the center of the circle is ( 6, 21) and its radius is 4.
This information is enough to write the equation.
(x- 6)^2+(y- 21)^2= 4^2
⇕
(x-6)^2+(y-21)^2=16
