Personally, I would save up and pay for the computer. If I need the computer right away or for a good reason, yes I will use a credit card to buy the computer.
On the other hand, if I don't need it at the moment, I will save up to buy it later and would probably save anyway because it's easier than stacking up on my credit card depending on how much I use my credit card.
(This was a quickly made response, adjust it anyway you would like and if it doesn't fit the question than it is perfectly OK to not use it!)
The answer is 14. This is because 0.30 multiplied by 5 is 1.5. Then 6 multiplied by 0.25 is 1.5 as well. Adding these two together is 3. 3 plus 3 for the notebook is 6. Than 6 plus 8 dollar tax is 14 dollars.