Answer:
u² + 4u - 21
Explanation:
given
(u - 3)(u + 7)
each term in the second factor is multiplied by each term in the first factor, that is
u(u + 7) - 3(u + 7) ← distribute parenthesis
= u² + 7u - 3u - 21 ← collect like terms
= u² + (7u - 3u) - 21
= u² + 4u - 21