Answer :
The answer is (3x + 7)².
(a + b)² = a² + 2ab + b²
9x² = (3x)² = a² ⇒ a = √a² = √(3x)² = 3x
49 = 7² = b² ⇒ b = √b² = √(7)² = 7
(3x + 7)² = (3x)² + 2 * 3x * 7 + 7² = 9x² + 42x + 49
(a + b)² = a² + 2ab + b²
9x² = (3x)² = a² ⇒ a = √a² = √(3x)² = 3x
49 = 7² = b² ⇒ b = √b² = √(7)² = 7
(3x + 7)² = (3x)² + 2 * 3x * 7 + 7² = 9x² + 42x + 49