`a) A = 3x^2 . (x+1) - 2x . (x^2 + 3) - x^2 . (x+3)`
` = 3x^3 + 3x^2 - 2x^3 - 6x - x^3 - 3x^2`
` = (3x^3 - 2x^3 - x^3) + (3x^2- 3x^2) - 6x`
` = 0 + 0 - 6x`
` = -6x`
`b) B=(3x-1).(2x+5)-(3x+2).(2x+3)`
` = (6x^2 + 15x - 2x - 5) - (6x^2 + 9x + 4x + 6)`
` = 6x^2 + 15x - 2x - 5 - 6x^2 - 9x - 4x - 6`
` = (6x^2 - 6x^2) + (15x - 2x - 9x - 4x) - (5+6)`
` = -11`
`c) C=2.(x+5).(2x+1)-(4x-3).(x+6)`
`= (2x + 10).(2x+1) - (4x^2 + 24x - 3x - 18)`
` = 4x^2 + 2x + 20x + 10 - 4x^2 - 24x + 3x + 18`
` = (4x^2 - 4x^2) + (2x+ 20x - 24x + 3x) + (10+18)`
` = x + 28`
`d) D=x.(2x+3).(2x-3)-4x.(x+3).(x-3)`
` = (2x^2 + 3x).(2x-3) - 4x.(x^2 - 9)`
` = 4x^3 - 6x^2 + 6x^2 - 9x - 4x^3 + 36x`
` = (4x^3 - 4x^3) + (6x^2 - 6x^2) + (36x - 9x)`
` = 27x`
