x^4-(a^2+9)x^2+9a^2=0

(x^2-9) (-(a-x)) (a+x) = 0

(x^2-9) (-(a^2-x^2)) = 0

x = -3

x = 3

x^4-(a^2+9)x^2+9a^2=0

x^4-х^2*a^2-9x^2+9a^2=0

(x^4-9x^2)-(х^2*a^2-9a^2)=0

x^2(x^2-9)-a^2(x^2-9)=0

(x^2-9)(x^2-a^2)=0

x^2-9 =0   или   x^2-a^2=0

х = ±3                  х=±а