4x4 + 13x2 + 36 is a quadratic in x2.
The discriminant is 132 - 4*4*36 = -407.
Since the discriminant is negative, the expression has no real roots and so it cannot be factorised (in the normal sense of the term).
(x - 2)(x + 2)(x - 3)(x + 3)
36x1 13x2 12x3 9x4 6x6
x(x2 + 36)
x2 + 13x + 36 = (x + 9)(x + 4)
x2 + 36 cannot be factored. You can only factor the difference of two squares, not the sum.
(x + 4)(x + 9)
(x + 9)(x + 4)
2(x + 6)(x + 3)
(x-4)
(5w-6)2
(6 - 5x)(6 - 5x)
(x - 6)(5x - 6)