Square of 9x – 7xy is

(c) 81x2 + 49x2y2 –126x2y

As per the condition in the question, (9x – 7xy)2

The standard identity = (a – b)2 = a2 – 2ab + b2

Where, a = 9x, b = 7xy

Then,

(9x – 7xy)2 = (9x)2 – (2 × 9x × 7xy) + (7xy)2

= 81x2 – 126x2y + 49x2y2