Declare a var for cx and cy, centre of circle.
In setup(), set their initial values.
In draw(), increase cx by 1
Then check if the circle has reached the right edge.
A first attempt might be if (cx >= width){
But that tests if the circle centre is at the edge.
An extra diam/2 allows the whole circle to move out.
cx = 0 sets the circle centre to the left edge.
A first attempt might be to set cx = 0
Subtract diam/2 for the whole circle to appear gradually.