NEED HELP WITH iii, MATLAB portion, how do I account for negative index?

Delta is for the delta function, it is called dirac in MATLAB, it has the value 0 for all x 0, and for x = 0.

For each of the following four system impulse responses: i) Find the system response, y[n], and describe in words any interesting features of the system. e.g. "It's a causal running sum filter". If the system is causal, draw a block diagram of the system. ii) Calculate and sketch the output given: x[n] = delta [n + 2] - 3 delta [n + 1] + 4 delta [n] - 3 delta [n - 1] + delta [n - 2]. iii) Use Matlab to plot ('stem') the input x[n] and output y[n] on the same axis for sufficiently informative time values, given input signal: x[5 lessthanorequalto n lessthanorequalto 45] = cos (n pi/10): else x = 0. Write a single script to plot the four answers (a-d) in separate subplots, and publish your results, or include the code. a) h[n] = 5 delta [n - 5] b) h[n] = 2 delta [n + 1] - 2 delta [n] c) h[n] = delta [n] - 2 delta [n - 1] + delta [n - 2] d) h[n] = 1/5 sigma^2_k = -2 delta [n - k]