Why are the Israelites to eat no leavened bread or have yeast among all of their borders for seven days?

They are to eat no leavened bread or have yeast because they came out of the land of Egypt in haste.

What are the Israelites to do with any meat that they sacrifice in the evening on the first day?

The Israelites are to get rid of any of the meat that they sacrifice on the evening of the first day so that it does not remain until the morning.