Question 378944
Use a substitution.
Let {{{u=x^2}}}, then {{{u^2=x^4}}}
{{{u^2-3u-54=0}}}
{{{(u-9)(u+6)=0}}}
Two solutions:
{{{u-9=0}}}
{{{u=9}}}
{{{x^2=9}}}
{{{highlight(x=0 +- 3)}}}
.
.
.
{{{u+6=0}}}
{{{u=-6}}}
{{{x^2=-6}}}
{{{highlight(x=0 +- sqrt(6)i)}}}