ELLE ~ 
on your success so far; 50 lbs is awesome!
Uhmmm ... for your belly ... continue to do some crunches and situps every day; you don't have to do a lot of them, just try to do some at least 6 days a week. Leg lifts work the bottom of the tummy as well. You can also google abdomenal exercises to get some more ideas. You can lose 1-2 sizes or more, just by adding these while you continue to lose weight. If you have had children, there may be some extra skin left once you reach your goal weight, but you can hide that with proper fitting clothes (meaning not too tight) when you reach your goal weight.