Question 300764
Make a substitution.
Let {{{u=y^2}}}, then {{{u^2=y^4}}}
{{{ y^4-8y^2-9 =u^2-8u-9}}}
{{{y^4-8y^2-9=(u+1)(u-9)}}}
Now substitute back,
{{{y^4-8y^2-9=(y^2+1)(y^2-9)}}}
Both can be factored as a difference of two squares,
{{{y^4-8y^2-9=(y+i)(y-i)(y-3)(y+3)}}} where {{{i=sqrt(-1)}}}