You can’t unless you are told the profit margin (or mark-up %).
Then you can calculate the cost of sales. Once you know the cost of sales, then you use the fact that cost of sales is equal to opening inventory plus purchases minus closing inventory. If you know the opening and closing inventories you can then calculate the purchases.
Have you watched the free lectures? The lectures are a complete free course for BA3 and cover everything needed to be able to pass the exam well.
(We stopped using the word ‘stock’ many years ago – it is called ‘inventory’.)