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)
Hope this helped :)
