\[\left( x^{4} \right)^{3} \cdot \ *\ = - x^{15}\]
\[x^{12} \cdot \ *\ = - x^{15}\]
\[*\ = - x^{15}\ :x^{12}\]
\[* = - x^{15 - 12}\]
\[*\ = - x^{3}.\]