The Factor Theorem states that the Polynomial P(x) has (x-r) as a factor if and only if P(r) = 0
Use r = -1
P(x) = x^4 + x^3 + x^2 + x + 1
P(r) = x^4 + x^3 + x^2 + x + 1
P(-1) = -1^4 + -1^3 + -1^2 + -1 + 1
P(-1) = 1 + -1 + 1 + -1 + 1
P(-1) = 3 - 2
P(-1) = 1
Therefore, (x+1) is not a factor of x^4 + x^3 + x^2 + x + 1