Question 969612
I would factor into 
(x^2-5)(X^2-4)=0.  It is just like factoring a quadratic, only you end up with quadratic roots.

x^2-5=0   x^2=5   x=plus sqrt(5) and minus sqrt (5)

x^2-4=0
(x+2)(x-2)=0  Difference of squares.

x=2,-2

The four roots are -sqrt (5), -2, 2, and sqrt (5)

{{{ graph(300,200,-3,3,-5,30,x^4-9x^2+20)}}}