If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Head over to the vent ramp and participate in a big air contest with Scooby Doo.
Start Scooby's motion until he can get off the ramp and when he is airborne, increase his speed and height, so is his downward speed. Repeat the steps to jump higher and higher. Your sense of coordination will help Scooby do the big air. Start rolling!