Respuesta :

sqdancefan
You can factor this by grouping.
  (x³ + 5x²) -(4x +20) = x²(x+5) -4(x+5)
  = (x² -4)(x +5)
Using the factored form of the difference of squares, this is
  = (x -2)(x +2)(x +5)

Zeros are the values of x that make these factors be zero.
  x ∈ {-5, -2, 2}