\(\left(x^2-y^2+6x+9\right):\left(x+y+3\right)\)

\(=\text{[}\left(x^2+6x+9\right)-y^2\text{]}:\left(x+y+3\right)\)

\(=\text{[}\left(x+3\right)^2-y^2\text{]}:\left(x+y+3\right)\)

\(=\left(x+3+y\right)\left(x+3-y\right):\left(x+y+3\right)\)

\(=x+3-y\)

_{1, \(x^2-y^2-x-y=\left(x-y\right)\left(x+y\right)-\left(x+y\right)\)}

_{=\(\left(x+y\right)\left(x-y-1\right)\)}

2, \(x^4-5x^2+4=x^4-4x^2-x^2+4\)

\(=x^2\left(x^2-4\right)-\left(x^2-4\right)\)

\(=\left(x^2-4\right)\left(x^2-1\right)\)

\(=\left(x-2\right)\left(x+2\right)\left(x-1\right)\left(x+1\right)\)

3, \(25-x^2+2xy-y^2=5^2-\left(x^2-2xy+y^2\right)\)

\(=5^2-\left(x-y\right)^2\)

\(=\left(5-x+y\right)\left(5+x-y\right)\)