`21) 9x^2 - 6x + 1 - 16y^2`

`= ( 3x - 1 )^2 - ( 4y )^2`

`= ( 3x - 4y - 1 )( 3x + 4y - 1 )`

`22) x^3 - 2x^2 - 9x + 18`

`= x^2( x - 2 ) - 9( x - 2 )`

`= ( x - 2 )( x^2 - 9 )`

`= ( x - 2 )( x - 3 )( x + 3 )`

`23) 4x^2 - 8x + 3`

`= 4x^2 - 6x - 2x + 3`

`= 2x( 2x - 3 ) - ( 2x - 3 )`

`= ( 2x - 1 )( 2x - 3 )`

`25) x^4 - 10x^2 + 9`

`= x^4 - x^2 - 9x^2 + 9`

`= x^2( x^2 - 1 ) - 9( x^2 - 1 )`

`= ( x^2 - 1 )( x^2 - 9 )`

`= ( x - 1 )( x + 1 )( x - 3 )( x + 3 )`

`24) x^2 + 2xy + y^2 - 25z^2`

`= ( x + y )^2 - ( 5z )^2`

`= ( x + y - 5z )( x + y + 5z )`

`26) x^3 + x^2y - 4x - 4y`

`= x^2( x + y ) - 4( x + y )`

`= ( x + y )( x^2 - 4 )`

`= ( x + y )( x - 2 )( x + 2 )`

`27) 3x^2 + 5x - 2`

`= 3x^2 + 6x - x- 2`

`= 3x( x + 2 ) - ( x + 2 )`

`= ( x + 2 )( 3x - 1 )`

`28) ( x^2 + 2x )^2 - 11( x^2 + 2x ) + 24`

`= ( x^2 + 2x )^2 - 3( x^2 + 2x ) - 8( x^2 + 2x ) + 24`

`= ( x^2 + 2x )( x^2 + 2x - 3 ) - 8( x^2 + 2x - 3 )`

`= ( x^2 + 2x - 8 )( x^2 + 2x - 3 )`

`= ( x^2 + 4x - 2x - 8 )( x^2 + 3x - x -3 )`

`= ( x - 2 )( x + 4 )( x - 1 )( x + 3 )`

`29) x^3 - 6x^2 + 9x - xy^2`

`= x( x^2 - 6x + 9 - y^2 )`

`= x[ ( x - 3 )^2 - y^2 ]`

`= x( x - y - 3 )( x + y - 3 )`

`30) x^4 + 324`

`= x^4 + 36x^2 + 324 - 36x^2`

`= ( x^2 + 18 )^2 - ( 6x )^2`

`= ( x^2 - 6x + 18 )( x^2 + 6x + 18 )`