from what i understand:-

1. You borrow 10 units stock.
2. Then you sell the 10 units stock at current market price RM10 per unit. So you get RM100.
3. Stock price drop to RM7.
4. You need to return the 10 units stock. Since you already sold it, so you need to buy back.
5. You buy back at RM7 per unit, so total to spend is RM70 for 10 units.
6. Profit = RM100 - RM70 = RM30