`a, x^2 - 3x + xy - 3y`
`= x ( x - 3 ) + y ( x - 3 )`
`= ( x - 3 ) ( x + y )`
`b, 16 ( 2x + 3 )^2 - 9 ( 5x - 2 )^2`
`= [ 4 ( 2x + 3 ) ]^2 - [ 3 ( 5x - 2 ) ]^2`
`= ( 8x + 12 )^2 - ( 15x - 6 )^2`
`= [ ( 8x + 12 ) - ( 15x - 6 ) ] [ ( 8x + 12 ) + ( 15x - 6 ) ]`
`= ( 8x + 12 - 15x + 6 ) ( 8x + 12 + 15x - 6 )`
`= ( -7x + 18 ) ( 23x + 6 )`