Respuesta :

5x⁴ - 3x³ + 6x) - (3x³ + 11x² - 8x)

Expand the second bracket by multiplying throughout by -1
5x⁴ - 3x³ + 6x - 3x³ - 11x² + 8x

Group like terms and simplify 
5x⁴ - 3x³ - 3x³ - 11x² + 6x + 8x

5x⁴ - 6x³ - 11x² + 14x
14x - 20 <------------------ answer.
Hope this helps!