That's to pass Mars slowly . You can get there much quicker if you don't mind whizzing past at high speed, with the same initial energy as if you were doing the slow route

It's not going anywhere near Mars. And it's dead now, no ability to manuever

Really not even close to the asteroid belt. It will make it just past Mars orbit.....right??

Correct

So these numbers are assuming that the C3=12 was correct? Either the C3 is correct, or the distance from the sun is correct, but not both?

We now know that the C3=12 was correct and the 'aphelion in the belt at 2.6 AU' was a mistake.

You know a lot, so I have a question: Musk advertised the car as being on a Mars-orbit trajectory. After the final burn. he bragged that it was going past that. Isn't this cause for concern? If Cassini went past Saturn no one would be bragging.

He was speaking loosely, it was never targeted precisely for Mars. And the going past that brag turned out to be a mistake, it is in fact going just a smidge past Mars orbit

I want to understand the calculation a lot better ...where should I start....as in what is C3?? And what formula is used etc..

search on 'astrodynamics'. We split the problem up into Kepler's-laws-like problems in the near-Earth and far-from-Earth cases. Near Earth it's a hyperbola, and C3 is velocity-squared it would have at infinity if the rest of the universe didn't exist