= 81x^2 - (9x + 7x)^2= 81x^2 - (16x)^2= 81x^2 - 256x^2= -175x^2
Therefore, the simplified expression is -175x^2.
= 81x^2 - (9x + 7x)^2
= 81x^2 - (16x)^2
= 81x^2 - 256x^2
= -175x^2
Therefore, the simplified expression is -175x^2.