A difference of perfect squares, x2 - a2, is always obtained from identical binomial factors except for the sign. The first term of each factor is the square root of the first term of the original binomial and the second term of each factor is the square root of the second term of the original binomial.

(x - a)(x + a)

EXAMPLE: 36x2 - 49

(6x)(6x) = 36x2

(7)(7) = 49

HENCE THE FACTORS ARE: (6x + 7)(6x - 7)

NOTE: there are no real factors for a sum of squares.