Question 1208827
{{{t=x^2}}}


{{{x^4+3x^2-4=0}}}

{{{t^2+3t-4=0}}}

{{{(t-1)(t+4)=0}}}


{{{t-1=0}}}

{{{t=1}}}

{{{x^2=1}}}

{{{x=1}}}  or {{{x=-1}}}


or  {{{t+4=0}}}

{{{t=-4}}}

{{{t=2i}}}  or {{{t=-2i}}}



x can be -1, or 1, or -2i, or 2i.