Answer:
x³ + 9x² + 23x + 15
Explanation:
(x + 1)(x + 3)(x + 5) ← expand the second/third factors using FOIL
= (x + 1)(x² + 8x + 15)
each term in the second factor is multiplied by each term in the first factor, that is
x(x² + 8x + 15) + 1 (x² + 8x + 15) ← distribute parenthesis
= x³ + 8x² + 15x + x² + 8x + 15 ← collect like terms
= x³ + 9x² + 23x + 15