Voila :)
2x² + 6√2x + 9
= √2² × x² + 6√2 +9
= (√2x)² + 2 × √2x × 3 + 3²
= (√2x + 3)²
(4x+15)² - (6x+1)² + (x-7) × (x+1)
= 16x² + 120x + 225 - (36x² + 12x +1) + x² + x - 7x - 7
= 16x² + 120x + 225 - 36x² - 12x + 1 + x² + x - 7x -7
= -19x² + 102x + 217
