Ответ:

  1. \((x-3)^2-8=x^2-6x+9-8=\boxed{x^2-6x+1}\).
  2. \(12x-(x+6)^2=12x-(x^2+12x+36)=\boxed{-x^2-36}\).
  3. \((2a-3b)^2-4a(a-6b)=4a^2-12ab+9b^2-4a^2+24ab=\boxed{12ab+9b^2}\).
  4. \((2x-3y)^2+(4x+2y)^2=4x^2-12xy+9y^2+16x^2+16xy+4y^2=\boxed{20x^2+4xy+13y^2}\).
  5. \((x-5)^2-x(x+3)=x^2-10x+25-x^2-3x=\boxed{25-13x}\).
  6. \((6a-b)^2-(9a-b)(4a+2b)=36a^2-12ab+b^2-(36a^2+14ab-2b^2)=\boxed{3b^2-26ab}\).
  7. \(3x(5+x)^2-x(3x-6)^2=3x(x+5)^2-9x(x-2)^2=3x\bigl((x+5)^2-3(x-2)^2\bigr)=\boxed{-6x^3+66x^2-39x}\).
  8. \((x-2)^2+(x-1)(x+1)=x^2-4x+4+x^2-1=\boxed{2x^2-4x+3}\).
  9. \((3a-2b)(3a+2b)-(a-3b)^2=9a^2-4b^2-(a^2-6ab+9b^2)=\boxed{8a^2+6ab-13b^2}\).