You already know, that you must take in less calories than you burn, to lose weight. The belly area, especially for women (and most particularly women that have carried babies) is a very hard area to "firm up," after ANY amount of direct "belly exercise."
When I lost 125 pounds over a 2 year period, (no surgery, no special diet) I was left with what I call a "mud flap," that looks like it's hanging off an 18 wheeler.