Expand :

(i) (x - 2y + 6)(x - 2y - 6).

(ii) (2a - 4b + 7) (2a + 4b + 7).

(i) (x - 2y + 6)(x - 2y - 6).

= x $\times$ (x - 2y - 6) - 2y $\times$ (x - 2y - 6) + 6 $\times$ (x - 2y - 6)

= x² - 2xy - 6x - 2xy + 4y² + 12y + 6x - 12y - 36

= x² + 4y² - 4xy - 36

Hence, (x - 2y + 6)(x - 2y - 6) = x² + 4y² - 4xy - 36.

(ii) (2a - 4b + 7) (2a + 4b + 7).

= 2a $\times$ (2a + 4b + 7) - 4b $\times$ (2a + 4b + 7) + 7 $\times$ (2a + 4b + 7)

= 4a² + 8ab + 14a - 8ab - 16b² - 28b + 14a + 28b + 49

= 4a² + 28a - 16b² + 49

Hence, (2a - 4b + 7) (2a + 4b + 7) = 4a² + 28a - 16b² + 49.