How do I make this code. This is difficult for me I am stuck in deque.h file

While we are more than willing to help those that are stuck, that doesn't mean that we are here to do it all for you! We can't do all the work, you are either getting paid for this, or it's part of your grades and it wouldn't be at all fair for us to do it all for you.

So we need you to do the work, and we will help you when you get stuck. That doesn't mean we will give you a step by step solution you can hand in!
Start by explaining where you are at the moment, and what the next step in the process is. Then tell us what you have tried to get that next step working, and what happened when you did.

Just a copy'n'paste of your assignment and "I tried" isn't going to get you anywhere.

If you are having problems getting started at all, then this may help: How to Write Code to Solve a Problem, A Beginner's Guide[^]

Quote:

Should done using Stack.h - Standard Template Library
Should done using Deque.h - Standard Template Library
Should done using Deque.h - Standard Template Library

C++ headers usually do not have an .h extension.
https://en.cppreference.com/w/cpp/container/stack
https://en.cppreference.com/w/cpp/container/deque
Is the third requirement duplicated, or should there be another requirement here?

As Rick has already suggested, it would make sense to first create some examples and then to decompose them. While decomposing check what should be on the stack first and how arithmetic operations and if necessary brackets have an effect.

Examples:

Infix:  A*B+C
Postfix: AB*C+
 
Infix:  (A+B)*(C/D)
Postfix: AB+CD/*
 
Infix:  A*(B*C+D*E)+F
Postfix: ABC*DE*+*F+
 
Infix:  (A+B)*C+(D-E)/F+G
Postfix: AB+C*DE-F/+G+