Calculating the rate of change over time

I am attempting to use Excel to track my ongoing weight loss. I have an artificial heart valve and my cardiologist has advised me not to lose more than 1 pound a week lest I put strain on my valve.

In my spreadsheet:

Column A is dates. (I put the date I weigh myself here.)
Column B is my weight in pounds.

I would like to know my average weekly change in weight (in pounds).

Any help you can provide would be greatly appreciated.