Your line
y(n)-y(n-1)+0.9*y(n-2)=x(n)+ x(n-1);
is not valid syntax. The left hand side of an assignment needs to be a reference to a location, not an expression. The line does not match your title of the Question, where you have
y(n)=y(n-1)-0.9*y(n-2)+x(n)+x(n-1)
Remember, MATLAB input is commands not algebraic expressions.
You could probably write it as a filter of some kind, but for now write it as a for loop:
x = y;
Y = zeros(size(x));
Y(1:2,:) = randn(2,size(x,2));
for n = 3 : size(x,2)
Y(n,:) = Y(n-1,:) + 0.9 * Y(n-2,:) + x(n,:) + x(n-1,:);
end
If you look at this carefully you will see that x(1,:) is never used and that Y has some random initializations and so will produce different results every run. These things happen when you do not define your boundary conditions.
