c++ on knoppix

I'm new to the whole knoppix scene, but i have an assignment that requires compiling and running a c++ program on knoppix, but everytime i try to compile it using g++ commmand, i get the xlib not found error, so i downloaded the debian package xfree86 and tried to install it, but it won't be installed, can anyone tell me how to do that?
I tried that on knoppix 5, 3.6 and 3.9 and they all give me the same thing.

thanks in advance.