If the square of (y-1)x +(y+ 1)x+9=0 is a linear equation about x, y=?

x？ Coefficient =0, square of y-1 =0, y= 1 or y =-1;

The coefficient of x is not equal to 0;

Y+ 1 is not equal to 0, and y is not equal to-1;

So y =1;