Perfect Lava Mixture

lava mixture puzzle
Bowser is making the perfect lava mixture to conquer the castle and capture princess Peach. He has a 36 gallon cauldron filled with lava that is 65% fire flower and 35% poisonous mushroom extract. He wants his lava mixture to be 38% poison mushroom extract. So, he is going to add "x" gallons of pure poisonous mushroom extract to his mixture.

How many gallons of the lava mixture will he have once he achieves the perfect concentration?