Try this:
% Define number of elements.
numberOfElements = 30; % Can be 3 or whatever you want.
x=linspace(1,9, numberOfElements)
% Find index where x = 5
index5 = find(x <= 5, 1, 'last')
% Assign A = 5 up until x = 5.
A(1:index5) = 5;
% Assign A = 5 up until x = 5.
A(index5+1 : numberOfElements) = 6;
% Now plot it.
plot(x, A, 'b*-', 'LineWidth', 2);
grid on;
I tried to make it flexible and general. Of course it could also be made a one-liner (and I'm sure someone will do that) if don't use comments, hard code in numberOfElements, don't plot anything, etc. but I like to write code that is a little more flexible and robust. I hope it wasn't your homework - you should have been honest and applied a tag of homework if it was.
