Help Me Please.

      √[ (x+y)(x²- y²) ]

$=\\ \quad\sqrt{(x+y)(x^2- y^2)}\\ =\\ \quad\sqrt{(x+y)(x+y)(x-y)}\\ =\\ \quad\sqrt{(x+y)^2(x-y)}\\ =\\ \quad\sqrt{(x+y)^2}\sqrt{(x-y)} \\ =\\ \quad\left | x+y \right |\sqrt{x-y}$