This is a difference equation. Simplest way is to use a for-loop
u = ones(100, 1); % using constant input;
e = rand(101, 1); % generating a random error vector
y = zeros(101, 1); % pre-allocating output vector
y(1) = 0;
y(2) = 0; % initial conditions
for t = 3:numel(y)
y(t) = u(t-1)+u(t-2)+e(t)-y(t-1)-y(t-2);
end
