How much did it cost summer to to buy 2.6 pounds of granola at the old price

It was $6.00 per pound.  So 2 pounds would be $12.  But there is still that .6 of a pound left.  If we know 1 pound costs $6, then we know 1/10th of a pound will cost $0.60.  So 6 x .60 = 3.60.  Add 12 and 3.60, and you get 15.60 for 2.6 pounds of granola at the old price.

17.94
because 15% of $6 is .9 so 2.6x6.9=17.94