In simplifying $$[3a(b-c)+5][-3a(b-c)-5],$$ I used $$4(au+bv)(cu+dv)=acu^2+(ad+bc)uv+bdv^2.$$
I failed to apply the formula to the equation because $a=3a$, $b=-3a$, $c=5$, $d=-5$, $u=(b-c)$, $v=?$
There's no value of $v$ so I tried to find other special product but it's only
$$(au+bv)(cu+dv)=acu^2+(ad+bc)uv+bdv^2$$ fit to the equation.