2xdiameter

var y1 = 50, x = 0;
function setup() {
  createCanvas(400, 400);
  frameRate(1);
}
function draw() {
  num =  pow(2,x);
  fill(random(255),random(255),random(255));//randomize color
  circle(200,y1,num)//draw circle with 2^x diameter
  text('2',x*20,12);//write 2 and have it move left with each circle
  text('x',x*20-10,12);//write x and have it move left with each circle
  fill(255)//make a white rectangle diameter text
  rect(330,0,100,20)//rectangle for diameter text
  fill(0)//make the text black
  text(str(num),350,12)//diamter text box
  x = x+1;//increase the exponent by 1
  y1 = y1+12;//move the center of the circle down 12 
  if(x == 11){//make sure that the animation loops and doesn't crash
    x = 0;//reset x
    y1 = 100;//reset y
    background(255)//reset background
  } 
}