Let's break it down into pieces.
First, write a program that prompts a user to enter "4.2", store it in a double, and then print the entered value back out.
There is a tutorial on the site here: https://cplusplus.com/doc/tutorial/basic_io/
And a variety of off-site resources.
Read the above link and try it out for yourself.
From your previous post, it looks like you know how to get input from the user, and you know about arrays.
So what specifically about this
challenge is giving you an issue?
For the GWA, first make sure you understand how to calculate it by hand.
Then, you just need to codify it. Show us your attempt.