It seems that your answer is '234561234561' but the good answer is "234561876549" (read the description where this example is shown). The test compares your answer (called "actual" in the test) and the good answer (called "expected" in the test). The test doesnt' give the parameter that is used. If you want to know it use a "print(strng)" in your code. Google "python assertequal" and you will have more explanations.