There are a lot of "bugs" in your code.
Frist, you miss the using namespace std; line before the main() function, so the compiler might not recognize cout and string .
You also missed a semicolon after "Hello World!" .
Another thing is that you can't declare variables inside cout , so you should take float x = 5.0f * str outside cout . The multiplication operation of a float type and string type is not even declared, so float x = 5.0f * str has no meaning. The variable 65Num has an "illegal" name - variables' names cannot begin with a digit.