In this form, ( h, k) is the center of the circle and r is its radius. Now, let's consider the following diagram.
We can see from the diagram that the center of the circle is ( 0, 0) and its radius is 4.
This information is enough to write the equation.
(x- 0)^2+(y- 0)^2= 4^2
⇕
x^2+y^2=16
