9x^4-x^2=0 step by step
factor out x^2 to get
x^2 (9x^2-1) = 0
Now you have a difference of squares
x^2 (3x-1)(3x+1) = 0
Now just read off the four roots.
x^2 (9x^2-1) = 0
Now you have a difference of squares
x^2 (3x-1)(3x+1) = 0
Now just read off the four roots.