I need help solving this using U- Substitution

Question

(x^2-4)^2 + (x^2-4) -6=0

Answer 1

u=x²-4
Equation becoms
u²+u-6=0
(u+3)(u-2)=0
so
u=2=x²-4 => x²=6 => x=±√6

u=-3=x²-4 => x²=1 => x=±1

Note: A quartic equation has 4 roots.