VS 2010 Returning the difference between two strings

I have done a quick search online and have had no luck so I thought I should post a thread here.

Can I please have some help to find and then return the difference between two strings.

E.g

String1:
This is a test line 1
This is a test line 2

String2:
This is a test line 1
Difference
This is a test line 2

What is the best way to return a string that informs me that the difference between the two strings is the line "Difference"?