\[5x + y^{3} - 2\left( x^{3} \right)^{5}y - 3x + 4x^{15}y =\]
\[= 2x + y^{3} - 2x^{15}y + 4x^{15}y =\]
\[= 2x^{15}y + y^{3} + 2x.\]