`25,x^2-y^2+9+6x`
`=(x^2+6x+9)-y^2`
`=(x+3)^2-y^2`
`=(x+3-y)(x+3+y)`
`26,x^2-2x-4y^2-4y`
`=(x^2-4y^2)-(2x+4y)`
`=(x-2y)(x+2y)-2(x+2y)`
`=(x+2y)(x-2y-2)`
`27,25-4x^2-4xy-y^2`
`=25-(4x^2+4xy+y)`
`=5^2-(2x+y)^2`
`=(5-2x-y)(5+2x+y)`
`28,(x^2+x)^2+(x^2+x)-6`
`=(x^2+x)^2+2.(x^2+x).(1)/2+(1/2)^2-25/4`
`=(x^2+x+1/2)^2-(5/2)^2`
`=(x^2+x+1/2-5/2)(x^2+x+1/2+5/2)`
`=(x^2+x-2)(x^2+x+3)`
