For a circle draw an ellipse
ellipse(right, down, width, height);
To locate it at the centre of the canvas:
• right is set to half of the canvas width
• down is set to half of the canvas height
Note that width and height are special names.
For a circle, the width and height are the same.
It's handy to create a var with a name like diameter.
In this example, values could be used directly:
ellipse(200, 150, 48, 48);
But using variables will be better.