Verified answer

[tex]\displaystyle\bf\\(x^{2} -1)^{2} -6(x^{2} -1)+9=0\\\\x^{2} -1=m\\\\m^{2} -6m+9=0\\\\(m-3)^{2} =0\\\\m-3=0\\\\m=3\\\\x^{2} -1=3\\\\x^{2} =4\\\\x_{1,2} =\pm \ \sqrt{4} =\pm \ 2\\\\Otvet \ : \ x_{1} =-2 \ \ ; \ \ x_{2} =2[/tex]