Implicit Cost

What is the implicit cost of not taking the discount on a 3/15 net 60 agreement?

theres a formula, let me take a look, cost of trade credit = (1+ .03/.97)^365/45 - 1 = 28.0% then take out the cost of taking the discount (1+ .03/.97)^365/15 - 1 = 109% so 81%? wtf. I dont know how to do this.