x^4-(9a^2+4)x^2+36a^2=0 помогите!

x²=n
n²-(9a²+4)n+36a²=0
D=(9a²+4)²-4*36a²=81a^4+72a²+16-144a²=81a^4-72a²+16=(9a²-4)²
n1=(9a²+4-9a²+4)/2=4⇒x²=4⇒x=-2 U x=2
n2=(9a²+4+9a²-4)/2=9a²⇒x²=9a²⇒x=-3a U x=3a