From the tiny picture it doesn't look like a mitered cut . Just looks like a straight cut to me //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
The parallax of the picture makes it difficult to tell I'm pretty sure that it is a mitered cut on just the one panel (sloped side of a hatchback style box). I use the same tool I use for every other cut: table saw.
It looks slightly slanted to me. I can see it by looking at the actual red outline of the edge of the panel. Easiest way to cut angles is on the table saw.