Question

using a suitable identity (7x -9y) (7x -9y)

Answer 1

The identity is
(a-b)^2 = a^2 - 2ab + b^2
(7x-9y)^2
(7x)^2 - 2×7x×9y + (9y)^2
49x^2 - 126xy + 81y^2

Answer 2

identity used=(a-b)^2=a^2-2ab+b^2
=(7x)^2-2(7x)(9y)+(9y)^2
=49x^2-126xy+81y^2.