I love this for the idea, and that you remembered Googie, who is not a very well remembered Scooby Doo character. I also love the attention to detail and the lighting hue that make it obvious its movie night at Shaggy's house despite the fact that the television is not shown. You stayed very true to the look of the characters, and Scooby seems especially accurate. His design is simple, but the line placement is key for him to look like himself. You did a great job with that.
However, I find that the composition lacks a bit. The characters are very small in the frame, and get swallowed up by the horizontal lines of the background, couch, and foreground. Cropping the picture closer to the characters will pull the focus in to them, and make it easier to see Velma's jealous, sneering expression. With so much empty space, the feeling of "Look out, she's behind you!" disappears because the eye can look so many other places. The empty space seems unnecessary for the work.
Nice job overall, and keep it up!