By using horizontal method,

We have;

= (x² – 5x + 8) × (x² + 2x – 3)

= x²(x² – 5x + 8) + 2x(x² – 5x + 8) – 3(x² – 5x + 8)

= x⁴ – 5x³ + 8x² + 2x³ – 10x² + 16x – 3x² + 15x – 24

By arranging the expression in the form of descending powers of x,

We get;

= x⁴ – 3x³ – 5x² + 31x – 24