That's a quadratic equation in [tex]x^2[/tex], which means we get an extra square root and [tex]\pm[/tex] at the end.
[tex]x^4 - x^2 - 42 = 0[/tex]
We factor,
[tex](x^2 - 7)(x^2+6) = 0[/tex]
[tex]x^2 = 7 \quad \textrm{or} \quad x^2 = -6[/tex]
[tex]x = \pm \sqrt{7} \quad \textrm{or} \quad x = \pm i \sqrt{6}[/tex]
