Regrouping the terms, we have

          6xy - 4y + 6 - 9x = 6xy - 4y - 9x + 6

                                  = 2y[3x - 2] - 3[3x - 2]

                                  = (3x - 2)[2y - 3]

∵   a⁴ - 2a²b² + b⁴ = (a²)² - 2(a²)(b²) + (b²)²

                                   = (a² - b²)²

                          = [(a² - b²) (a² + b²)]

                          = [(a - b) (a + b) (a² + b²)]

∴    a⁴ - 2a²b² + b⁴ = (a - b)(a + b)(a² + b²)

∵   x⁴ - (y + z)⁴ = [x²]² - [(y + z)²]²

                       = [(x²) + (y + z)²] [(x²) - (y + z)²]
                                                
                                                     [Using a² - b² = (a+b)(a-b)]

We can factorise [x² - (y + z)²] further as

       x² - (y + z)² = [(x) + (y + z)][(x) - (y + z)]

                         = (x + y + z)(x - y - z)

∴      x⁴ - (y + z)⁴ = (x + y + z)(x - y - z)[x² + (y + z)²]

Using              a² - b² = (a - b) (a + b), we have

                     a⁴ - b⁴= (a²)² - (b²)²

                               =

                             

                              = (a + b)(a - b)
                              
                             

We have  p⁴ - 81 = (p²)² - (9²)²

Now using a² - b² = (a + b)(a - b), we have

         (p²)² - (9)² = (p² + 9)(p² -9)

We can factorise p² - 9  further as
              p² - 9 = (p)² - (3)²

                       = (p + 3)(p - 3)

∴           p⁴ - 81 = (p + 3)(p - 3)(p² + 9)