By using horizontal method,

We have;

(9x² – x + 15) × (x² – x – 1)

= 9x²(x² – x – 1) – x (x² – x – 1) +15(x² – x – 1)

= 9x⁴ – 9x³ – 9x² – x³ + x² + x + 15x² – 15x – 15

Putting equal power terms together, we get,

= 9x⁴ – 9x³ – x³ – 9x² + x² + 15x² – 15x + x – 15

= 9x⁴ – 10x³ + 7x² – 14x – 15