Simulate LTI Signal Output Signal

Can anyone help me with part b. of the question? I'm having a hard time using matlab to simulate the output of this LTI system. I can do this by hand and have attached the question/answer for anyone to help. Thank you for your assistance.

My code:

d = rand(1,50); x = 7.*d - 7.*(d-2); S = zeros(1,length(x)); for n = 4:length(x) S(n)= x(n)- x(n-1) + 2*x(n-3); end; S