Which equation represents a linear function A) y 9 - x B) y x2 plus 1 C) y x3 plus 5 D) y x - 9?

The equation that represents a linear function is A) ( y = -x + 9 ) and D) ( y = x - 9 ). Linear functions can be expressed in the form ( y = mx + b ), where ( m ) and ( b ) are constants. Options B) and C) represent quadratic and cubic functions, respectively, due to the presence of ( x^2 ) and ( x^3 ).