The center of the circle is at: $ C = \left(0, 0\right) $.
The radius of the circle is $ r = 5 $.
A circle with center at $ ( \color{red}{h}, \color{blue}{k}) $ and a radius of $ r $ has equation $ (x - \color{red}{h})^2 + (y - \color{blue}{k})^2 = r^2 $.
In this example our circle equation can be written as:
$$ (x - \color{red}{0})^2 + ( y - \color{blue}{ 0 })^2 = \left( 5 \right)^2 $$So, we have: $ \color{red}{h = 0 }$ , $ \color{blue}{ k = 0 }$ and $ r = 5 $.