Q.sturdy weeds
My son has some sturdy weeds under his deck that come through the boards. He cut them, poured kerosene gas and weed killer but it comes back even stronger. Need help.

Here are a couple of links with a few ideas. Using kerosine and gasoline is extremely dangerous and would not be recommended.
https://www.gardeningknowhow.com/special/organic/homemade-pet-friendly-weed-killer.htm
https://www.gardeningknowhow.com/plant-problems/weeds/how-to-kill-grass-naturally-kill-unwanted-grass-in-your-yard.htm