Process of Trading in a car with negative equity

mkatc89

I am shopping for a larger car for my family and I want to trade my sedan which I owe more money on than its worth and I need some help understanding the math for that process, in my research, it says that basically I pay the amount that I owe (amount owed is added to the price of new car) and the amount the car is worth is subtracted from the purchase price of the new vehicle. I've been to two different dealerships and in their numbers, they add the total amount of the loan on my current car, is that the actual way it's done or are they being shady?? Any insight would be greatly appreciated! 

kyfdx

Cost of new car
+ amount of old loan
= gross amount
- value of trade-in
= net amount of new loan

Assuming you aren't adding any cash down.