What exercise should i do to flatten my tummy if diastasis?

Not much: A diastasis is when the abdominal muscle is open; unfortunately there is no exercise that will correct this. Maintaining your right weight, eating low GI and low carb can help prevent fat from depositing there and bringing attention to that area.

Planks: Try planks. You hold yourself as if you were going to do push ups but don't go down. Hold as long as you can, then you can work on side planks to define the obliques.