Simple Script to Combine/Seperate Lines of Text

[Ben Minton]

Hi All

Was wondering how to write a simple script that performs the following 
task:

--------------------------------------------------

airncjrlfojqmxirnxplvmndrhqndj ... e (length n) combined with

qpmgktincirzpwncjftewnchdtecru ... f (length n) equals

motnciqaxhfyeklpgmchqicngiehfc ... a (length n)


and then 

motnciqaxhfyeklpgmchqicngiehfc ... a (length n) subtract

qpmgktincirzpwncjftewnchdtecru ... f (length n) equals

airncjrlfojqmxirnxplvmndrhqndj ... e (length n)

--------------------------------------------------
uses plain text alphabetic characters only, where initial string of 
characters is any integer n, upto 1000.

I can write this in a flow diagram but don't understand strings enough to 
do this.

Thanks, Ben, Australia