Problem Solving

Could someone explain #20 in the Math Bin 4, Princeton Review Cracking the GMAT 2008 edition? It starts off, "Figure ABCD...." In the answer explanation, where does the 49 come from in the equation x2 – 2xy + y2 = 49 after squaring statement 1?

thanks

The first statement is 1) x-y = 7 --> (x-y)2 = 49 --> x2-2xy+y2 = 49!

Sorry,not all of us on this forum have access to this book.Well,let me speak for myself.i don't have access to the book,so could you kindly please post the question so we can share?

For the people who don't have the Princenton book:

There's an image of a rectangle that comes with the question, I draw it myself and attached it so you could have a clear image of what you're asked.

Question: Figure ABCD is a rectangle with sides of length x centimeters and width y centimeters, and a diagonal of length z centimeters. What is the measure, in centimeters, of the perimeter of ABCD?

1) x-y = z

2) z=13

I guess I needn't to say this is a DS question.

Cheers