I know it is your homwork and appreciate that you have tried something.
Loop version:
x = linspace(0,20,20);
% ^^------ increase to get a smoother curve
y2=zeros(size(x)); % preallocate
for k=1:numel(x)
% x2 = x; ?? does nothing
y2(k) = 20000*log(x(k))-3*x(k);
% ^^^------ save y2 in each iteration
end
plot(x,y2,'r') % plot outside the loop
Matlab's way (vectorization):
x = linspace(0,20,20); % just three lines of code
y2 = 20000*log(x)-3*x;
plot(x,y2,'r')
