Answer
$x^2+36=0$
Work Step by Step
As we are given that solution set : {$-6i,6i$}
This implies that $x=-6i,6i$
As per zero-product reverse property, we have $(x+6i)(x-6i)=0$
Now, apply FOIL METHOD
$(x+6i)(x-6i)=0\implies x^2-(6i)^2=0$
or, $x^2+36=0$
