Because the compound is an oxide it contains only nitrogen and oxygen.
The % oxygen in the compound is 100-25.9% =74.1%
Remember in empirical formula we can treat the % as having 100g of the compound, there would be 25,9 g nitrogen and 74.1 g oxygen.
Steps in empirical formula
The total sales figure is =107%. Your sales before tax is =100%. Use the ratio relationship to fine the sales before tax.
6645.16/107 = x(sales before tax)/100
Solve for x gives you 6210.43!
Check you answer 7% of 6210.43 = 434.73.
Total with tax = 6645.16...
Rewrite fractions with the same denominator (preferably the lowest factor for both numbers. This is 18 in his case.
-5/6 + (-5/18) = -15/18 + (-5/18)
Now that we have done that we can add the numerators and place the answer over the denominator.
-15 + (-5) = -20/18