What am supposed to do the "x" part is throwing me off.

You need to replace 'x' with the actual size of the array (in bytes). You will need to use the formatting options available in the printf function call. You can read about that here

printf("Array real_numbers is %ld bytes");

That's the first part. %ld is a placeholder to let C know we are going to put a long, signed integer there (see link above for descriptions of the placeholders).

The second part is to pass printf the value that we want to put in that placeholder. printf takes any number of arguments. The first argument that you see above is the message format. All arguments that follow correspond to any placeholders that we have in the message format.

printf("Array real_numbers is %ld bytes", sizeof(real_numbers));

Hope that helps!