How does a trade-in works at a company?

My car is financed and still has 10000 to go. Lets say i wanna trade it in for a brand new car at ford which costs 31,500.
if they value my car for 8000 dollar, how would they calculate it? Will they take off 8000 of the original price and add 2000 dollars or would they just add 2000 dollars to the original price?

Added (1). 10000 *dollars*

Of course, I have no idea & not that anyone actually does the smart thing, but do as I've always done… Pay cash! Save til you can afford to. That's what I have always done. It's obvious people who finance new cars get tired of the car long before they're paid for & from the sounds of your question I'd have to say that first car never actually gets paid for when people simply trade in for more expensive cars therefore payments & on & on. Drive the one you have til it's paid for. Period. You'll live without a new car not to mention the second you drive it off the lot it's not a new car anymore it's a used one. Many advantages to paying cash one being you aren't having to make some stupid payment I can't even imagine every month plus you also do't need full coverage ins so your costs are that much less! If you can't resist then don't go shopping it's very easy really.
Not what you want to hear I know but think about it & maybe later in your life it'll hit you