\[x^{3}y + 4\left( x^{2} \right)^{4}y^{2} - 15x - 4x^{6}y^{8} =\]
\[= x^{3}y + 4x^{8}y^{2} - 15x - 4x^{6}y^{8} =\]
\[= - 4x^{6}y^{8} + 4x^{8}y^{2} + x^{3}y - 15x.\]