Sounds like a simple compare operation.
What assembly language/OS are you allowed to use if not Windows assembly?

You'll also have to show your attempt to program this, it is your assignment after all.

-To do so, figure out how to open a memory mapped file in your OS, and get a pointer to the 1st character. If your OS doesn't have support for memory mapped files, allocate memory and load the file into memory.

-After that, look up cmpsb and rep.cmpsb instructions to help you allong.