This is a technique for solving heavy equations by substituting terms in variables.
Example
(x^2 -5x -5)(x^2-5x+3) = 9
let a = x^2 -5x + 3
thus, it is:
a(a-8) = 9
a^2 - 8a - 9 = 0
(a - 9)(a+1) = 0
(x^2 -5x -6)(x^2 -5x -4) = 0
(x+0.854)(x-0.697)(x-4.303)(x-5.854) = 0