Help me with TNT + Obsidian

Where do i put the following stuff, to allow it to actually work? I want to be able to break obsidian with tnt.

<?php

declare(strict_types=1);

namespace BlobKing21\ObbyDestroyer;

use pocketmine\event\Listener;
use pocketmine\plugin\PluginBase;
use pocketmine\block\Block;
use pocketmine\block\Obsidian;
use pocketmine\block\BlockFactory;

class Main extends PluginBase implements Listener
{
public function onEnable()
{
BlockFactory::registerBlock(new MyObsidian(), true);
}
}

class MyObsidian extends \pocketmine\block\Obsidian
{
public function getBlastResistance() : float
{
return 7.5;//same as stone
}
}

 

Pm3 or Pm4?

 

PM 4