6x 4 + 9x 3 – 15x 2 – 10x 3 – 15x 2 + 25x + 8x 2 + 12x – 20 = 6x 4 – x 3 – 22x 2 + 37x - 20

Simplify:

6x⁴ + 9x³ – 15x² – 10x³ – 15x² + 25x + 8x² + 12x – 20

= 6x⁴ – x³ – 22x² + 37x - 20