not sure its the right place to write this here or not,but i'll try my luck.
been given an example of plotting a graph,writing this into the command prompt :

L = 31;
nn = 0:(L-1);
imp = zeros(L,1);
imp(1) = 1;
stem(nn,imp)

this gives a graphic display.

then i have a few examples to do, the first being x [n]= 0.9āˆ‚[n-5] range being 1ā‰¤nā‰¤20
could anyone giving a hand,using the example format, then il have a rough idea of trying the others.thanks

You may read one of matlab tutorials here or this official site. The given code is to generate a horizontal array (dimension 1xL) value from 0 up to L-1, and then store it in variable nn. Then create an vertical array size of Lx1 filled with all 0 except the first index (if I remember correctly, matlab use 1-index instead of 0-index). Then you plot it.

I am not sure about the āˆ‚ symbol in your equation, so can't really say much. What I am thinking is that use your n as horizontal array, compute each n to find out x[n] and put it in a vertical array. Then you plot it.

L = 20;
nn = 1:L;        # create array [1, ..., L]
x = zeros(L,1);  # create vertical array size of Lx1
for n = 1:L    # loop through n value to compute for x[n]
  x(n) = equation_blah_blah;
end
stem(nn,x);      # plot it

PS: I believe there is a better way to utilize Matlab's efficiency of array computation (parallel computation). I can't remember the syntax though.

well in my examples, its for an absolute beginner like myself,only starting off here. whats the imp? impulse or something like that?

No, they are all variable names. When you declare a vairable in Matlab, you simply start with a name and assign a value to it. I don't know how much programming concept you have. A Computer language is just a language with different syntax. If you get passed the syntax, everything is simple to understand.

L = 31;
# The above line means you declare a variable L and assign value 20 to it

nn = 0:(L-1);
# The above line means you declare a vairable nn and create a horizontal array
# containing values from 0 up to L-1 which is 30 [0, 1, ..., 30]

imp = zeros(L,1);
# The above line means you declare a variable imp, and then assign a vertical
array size of Lx1 (31x1) filled with value 0 in it.

imp(1) = 1;
# The above line means you assign value 1 to the first index of the variable.
# Matlab array index starts from 1, not 0.

stem(nn,imp)
# The above line is calling a function stem to plot a graph using
# nn variable values on the x-axis and imp variable values on the y-axis