You are not just told the cost, you are also told that the markup is 40% of the selling price. Let M be the markup. Then the selling price is 6.60+ M and 40% of that is (0.4)(6.6+ M). Since the markup is 40% of the selling price, 0.4(6.5+ M)= M. Solve that equation for M and the rest of the problem should be easy.