Question 370593
Direct substitution gives {{{P(-5) = (-5)^3 + 4*(-5)^2 - 8(-5) - 15 = 0}}}.  Thus the remainder after dividing {{{x^3 + 4x^2 - 8x - 15 }}}by x+5 is 0, meaning x+5 is a divisor or factor of {{{x^3 + 4x^2 - 8x - 15 }}}.