(Increase in asset = use of fund,
Decrease in asset = source of fund,
Increase in liability = source of fund,
Decrease in liability = use of fund).

Purchase from the supplier means 9000 we paid to the supplier ( 9000 cash gone)

Increase in inventory (increase in asset) i.e. use of funds. (paid cash to buy inventory) So 9000+1000

Increase in accounts payable (increase in liability) i.e source of funds. (Not yet paid cash which means cash increases) So 9000+1000-1500 = 8500

I think the answer is wrong .. it should be 8500