Replaceable parameter syntax for the console object in C#.

I am taking the O'Reilly C# Course 1 and it is asking for a replaceable parameter syntax and it is not very clear on what that means. Currently I used this:

double trouble = 99999.0009;
double bubble = 11111.0001;
Console.WriteLine(trouble * bubble);

Am I missing the meaning of replaceable parameter syntax? Can someone provide an example for what I am looking for?

Original question for the quiz: "Create two variables, both doubles, assign them numbers greater than 10,000, and include a decimal component. Output the result of multiplying the numbers together, but use replaceable parameter syntax of the Console object, and multiply the numbers within the call to the Console.WriteLine() method."