Overhead press with kneeling crunch.
Push-up with side planks
We all know planks do a lot for the abs, not to mention push-ups. Combine them and wow!
Row with leg Extension
This is major core work. Your core is the foundation of every move, and there's a lot of moves going on here!
By combining upper and lower body you're working on your abs. Make sure you tighten your stomach for each exercise to ensure no damage to your back/core. When you tuck in your stomach you also tighten important muscles that will help you achieve a flat tummy. Happy exercising!