`x^2 + 13x + 36`

   `= x^2 + 9x + 4x + 36`

  `= x(x + 9) + 4(x + 9)`

 ` = (x + 9) (x + 4)`   (Ans)

`x^4 + x^2 - 20 = x^4 + 5x^2 - 4x^2 - 20`

  `= x^2 (x^2 + 5) - 4(x^2 + 5) = (x^2 + 5)(x^2 - 4)`

  `= (x^2 + 5) (x^2 - 2^2) = (x^2 + 5) (x + 2) (x - 2)`

  `= (x + 2) (x - 2) (x^2 + 5)`   (Ans)

`a^2 - 30a + 216`

  `= a^2 - 18a - 12a + 216`

  `= a(a - 18) - 12(a - 18)`

  `= (a - 18) (a - 12)`  (Ans)

`x^6y^6 - x^3 y^3 - 6`

  `= x^6 y^6 - 3x^3 y^3 + 2x^3 y^3 - 6`

  `= x^3 y^3 (x^3 y^3 - 3) + 2(x^3 y^3 - 3)`

  `= (x^3 y^3 - 3) (x^3 y^3 + 2)`  (Ans)

`a^8 - a^4 - 2`

  `= a^8 - 2a^4 + a^4 - 2`

  `= a^4(a^4 - 2) + 1(a^4 - 2)`

  `= (a^4 - 2) (a^4 + 1)`   (Ans)