Sound Problem involving a path difference

[Matt1234]

Attached is the problem and solution for part B, can someone please explain how they obtain the equation for part B, i don't see why they take the root.

Thank you.

Homework Statement

[PLAIN]http://img153.imageshack.us/img153/8149/97578282.jpg

 

[Matt1234]

no longer required sorry.

note for those who ever use this thread: d= sqrt ((x2-x1)^2 + (y2-y1)^2)