26 is incorrect because it is a decimal approximation. When asked to simplify, you are almost always expected to give your answers exactly (i.e. not simply plug the numbers into your calculator, but rather manipulate them by hand to get a more "simple" number. You shouldn't use a calculator at all). Given that you were able to do the other problems, I assume you can do 26 by yourself. Once you have the correct answer, re-post it and I'll check it for you.