Indexing begins at 1, not 0 as in most programming languages. The index of the last element in a given dimension is end. You can use it in expressions: v(end - 1) etc. Use ranges to access sub-matrices: B = A(k:l,m:n). Subscripts and ranges can be built from any integer-valued expressions.
You will then be at the colon prompt. At the colon prompt, hit return and you'll get back the minus prompt. At the minus prompt, enter e zyyy:11 where z=x+3, then press return. You should get back zyyy:0011 25. Immediately following zyyy:0011 25. type D followed by a space. This should return 3C. on the same line.