The answer to the equation is 12.
Divide both sides by -3, and replace
with
. Then

Factorize the quadratic in
to get

which in turn means

But
for all real
, so we can ignore the first solution. This leaves us with

If we allow for any complex solution, then we can continue with the solution we ignored:

Check the picture below
get the volume of each, sum them up, that's the volume of the figure