Check your spacing in the first string and add a period to the end of the second string, A lot of the tests are really specific about formatting issues.
You have made a working string but you need to create the same string as is requested. Which is: "This is the start. " and "This is the end."
So, do not forget that empty spaces are also part of a string. The string that you have made, is not identical to the one that is being requested.