`a)2x(3x+1)-(x-3)(6x+2)`
`=6x^2+2x-(6x^2+2x-18x-6)`
`=6x^2+2x-6x^2-2x+18x+6`
`=(6x^2-6x^2)+(2x-2x+18x)+6`
`=18x+6`
`b)(2x-3)^2-(1+2x)(2x+1)+3(2x-3)`
`=4x^2-12x+9-(1+2x)^2+6x-9`
`=4x^2-12x+9-1-4x-4x^2+6x-9`
`=(4x^2-4x^2)+(-12x-4x+6x)+(9-1-9)`
`=-10x-1`
`c)(x+y-1)^2-2(x+y-1)(x+y)+(x+y)^2`
`=(x+y-1-x-y)^2`
`=[(x-x)+(y-y)-1]^2`
`=(-1)^2`
`=1`