A polynomial is of degree n if its largest term is x^n.
The polynomial
is of degree 3 (because x * x^2 = x^3) and has two parts: (x+4) and (5x^2+4x+20).
The first part x+4 has roots at x = - 4, the second part does not have any real roots (the discriminant is < 0 ); therefore, the only real roots this polynomial has is x =-4.