returns is correct but work do not pass

<?php

class Rooms {

  private $dimensions = array();

  public function getDimensions()
  {
      return $this->dimensions;
  }

  public function setDimensions($l,$w)
  {
    $this->dimensions[] = $l;
    $this->dimensions[] = $w;  
  }

}

class Render {

    public static function detailsKitchen($room)
    {
      return "Kitchen dimensions: " . self::displayDimensions($room->getDimensions());      
    }

    public static function displayDimensions($size)
    {
        $str = "";
        $str .= $size[0] . " x " . $size[1]; 
        return $str;
    }

}

  $room = new Rooms();
  $room->setDimensions(12,14);
  echo Render::detailsKitchen($room);

?>