How to get a tight stomach?

I've been shedding a lot of weight. I used to weigh 220 and now I'm at 169. I go to the gym every night and excersise for about 2 hours. I can see that my stomach is beginning to shrink a little But what I really wanna know is, how to I make my core firm? What excersises to you guys recommend? I want that section to be flat and firm. If you guys have any other tips to lose weight, I'd appreciate those as well :]
How to get a tight stomach?
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