Look at what you have done manually...
You've taken the value of
and added the value of
then subtracted the value of
I wonder what would happen if you did that in a SELECT statement...
select Id, Beginning_Inventory, Purchased_Inventory, Used_Inventory,
Beginning_Inventory + Purchased_Inventory - Used_Inventory as Total_Inventory
There are some examples of how MySQL works on this link - MySQL Tutorial - MySQL By Examples for Beginners
If you want to update the last column then the syntax is
UPDATE myTable SET Ending_Inventory = Beginning_Inventory + Purchased_Inventory - Used_Inventory;
Note that this is not good practice - just calculate the final inventory when you need it rather than storing extraneous information