var x1 = 0, y1 = 0, x2 = 400, y2 = 400; // initial ordered pairs

function setup() {
  createCanvas(400, 400); //create the canvas
  background(0); //black background
  frameRate (5); //frame rate or speed
}

function draw() {
  stroke(150, 250, 200) //linecolor
  line(x1,y1,x2,y2) //draw line
  y1 = y1+ 5; //change the y1 with each loop
  y2 = y2 - 5;//change the y2 with each loop
  slope = (y2-y1)/(x2-x1); //calculate slope
  strSlope = str(round(slope,1)); //round the slope and make it into a string
  string = join(['Slope =  ',strSlope ],'');//create the string to print;
  fill(255); //set the color to white for the rectangle
  rect(180,380,100,400);//make the rectangle (x,y,width,height)
  stroke(0);//set the text color to black
  fill(0);//set the text fill to black
  text(string,200,395);//print the text
  if(y1>400){ //stop loop condition
    noLoop();
     }
}