I just started learning "return" and i don't understand the concept!

I don't know how to make two parenthesis, I am Just stuck

This is the question:

1.Write a function called perimeterBox that returns the perimeter of a rectangle.

2.It should have two parameters.One formula for perimeter is length + length + width + width;

3.Call the function and pass in any value for length and width you like.

This is where I am stuck