I got an equation y = 2x ;

but I only wanted to plot the specific range x> 4 without adjusting the axis shown, how do I do that?

 

Here's a good site to look up :http://www.mathworks.com/help/techdoc/ref/plot.html

Your question is quite easy, so you can expect Matlab Help to have the answer.

 

x=4:100;
y=2*x;

plot(x,y)