I don't recognize the form x^18-y^18 and i dont know to factor it.

You could write the given difference as a difference of 2
squares:

x^18-y^18 = (x^9)^2 -
(y^9)^2

We know that the difference of 2 squares yields the
product:

a^2 - b^2 =
(a-b)(a+b)

(x^9)^2 - (y^9)^2 = (x^9 - y^9)(x^9 +
y^9)

But x^9-y^9 may be written as a difference o 2
cubes:

x^9-y^9 = (x^3)^3 -
(y^3)^3

We'll note x^3 = a and y^3 =
b

As you know, the formula of difference of squares
is:

a^3 - b^3 = (a-b)(a^2 + ab +
b^2)

We'll substitute a and b  and we'll
get:

x^9-y^9 = (x^3 - y^3)(x^6 + (xy)^3 +
y^6)

x^9-y^9 = (x-y)(x^2 + xy + y^2)[x^6 + (xy)^3 +
y^6]

x^18 - y^18 = (x - y)*(x^2 + xy +
y^2)*[x^6 + (xy)^3 + y^6]*(x^9 + y^9)