Question 1132089
It does make a difference when you add the tax, and the discount is always taken first, before the tax. 
0.90(300x)=270x dollars for x tires.  A 10% discount gives the amount discounted; the 90% is what it costs.
The total price is the 270x+(0.06)270x) or 1.06(270x) or f(x)=$286.20x

buy 4 tires and that is 1200 dollars.  Ten per cent discount is $120, so the total cost is $1080
6% of $1080 is $64.80, the tax.  The final cost is $1144.80
286.20*4=$1144.80