Use the Fac

By Factor Theorem, we know that,

If p(x) is a polynomial and a is any real number, then g(x) = (x – a) is a factor of p(x), if p(a) = 0

For checking (x+1) to be a factor, we will find f(–1)

⇒ f(–1) = 5 (–1)³ + (–1)² – 5(–1) – 1

⇒ f(–1) = –5 + 1 + 5 – 1

⇒ f(–1) = 0

As, f(–1) is equal to zero, therefore, g(x) = (x+1) is a factor f(x)