I can’t upload any working right now so I hope you’ll understand from an explanation, all it is is a hidden polynomial. You can say:
Let y=x^2 Then the equation becomes: y^2 - 18y + 81 Factorise that to give you: (y-9)(y-9) Or (y-9)^2 But remember, y=x^2. So the equation becomes: (X^2-9)(X^2-9)