Question 896219
{{{u=x^2}}}


{{{4u^2-5u-9=0}}}
{{{(4u-9)(u+1)=0}}}


EITHER {{{4x^2-9=0}}}
{{{4x^2=9}}}
{{{x^2=9/4}}}
{{{highlight(x=-3/2)}}} OR {{{highlight(x=3/2)}}}
OR
{{{x^2+1=0}}}
{{{x^2=-1}}}
{{{x=-sqrt(-1)}}}  or  {{{x=sqrt(-1)}}}
{{{highlight(x=-i)}}}  or  {{{highlight(x=i)}}}