Question

Let p=x^2+6

Answer 1

That's a bit of a nasty. Where ever you see (x^2 + 6) you put in p.
p^2  - 21 = 4(x^2 + 6)
p^2 - 21 = 4p
p^2 - 4p - 21 = 0

A <<<< answer.