Answer:
(x + 10)²
Step-by-step explanation:
Given
(x + 3)² + 14(x + 3) + 49
Use the substitution u = x + 3, then the expression can be written as
u² + 14u + 49 ← which is a perfect square trinomial
= (u + 7)² ← change u back into terms of x
= (x + 3 + 7)²
= (x + 10)²
