We have
[tex]\begin{gathered} \mleft(-3x^3+8y^5\mright)\mleft(-5x^6-7y\mright) \\ (-3x^3\cdot-5x^6)+(-3x^3\cdot-7y)+(8y^5\cdot-5x^6)+(8y^5\cdot-7y) \end{gathered}[/tex]Solving each multiplication we obtain the final result
[tex]15x^9+21x^3y+-40x^6y^5+-56y^6[/tex]
