Factoring by Using a Quadratic Form

Question

Problem: x^4 + 8x^2 - 20

What I have so far:
(x^2)^2 + 8(x^2) - 20
a^2 + 8a - 20

For some reason my textbook says to substitute the (x^2) with (a) as you can see in the las step. The next step after that is factoring [a^2 + 8a - 20].

Answer 1

x^4 + 8x^2 - 20
(x^2 - 2)(x^2 + 10)