`a, ( 2x+3)(2x-3) - (5x-1)^2`
`= (2x)^2 - 3^2 - [ (5x)^2 - 2.5x.1 + 1]`
`= 4x^2 - 9 - ( 25x^2 - 10x + 1)`
`= 4x^2 - 9 - 25x^2 + 10x -1`
`= -21x^2 + 10x -10`
`b, 2x(3x+1) + (x+3)(2x-3)`
`= 6x^2 + 2x + 2x^2 - 3x + 6x - 9`
`= 8x^2 + 5x -9`
`\color{green}{\text{@BerryLinh}}`