81 is the same as 3⁴.
Thus, we can write it as:
[tex](3x)^{4} - 3^{4}[/tex]
[tex]= [(3x)^{2}]^{2} - [3^{2}]^{2}[/tex]
[tex]= [(3x)^{2} - 3^{2}][(3x)^{2} + 3^{2}][/tex]
[tex]= (3x - 3)(3x + 3)[(3x)^{2} + 3^{2}][/tex]
[tex]= (3x - 3)(3x + 3)(9x^{2} + 9)[/tex]
