To mark up a product by 200%, you need to add 200% of the original cost to the original cost.

Here's how you can do it in Excel:

1. In a cell, enter the original cost of the product, which is $4.50 in your case.
2. In another cell, enter the markup percentage, which is 200% or 2.
3. In a third cell, enter the formula "=original cost*(1+markup percentage)" without the quotes. In this case, it would be "=4.50*(1+2)".
4. Press enter and you will get the result, which is $13.50.

So the formula you need is "=original cost*(1+markup percentage)" where "original cost" is the cell containing the original cost of the product and "markup percentage" is the cell containing the markup percentage.