Zooming and 2D Graphics

[Comernada]

Hello, I am writing a 2D graphics program to draw basic shapes and I was hoping someone could help me figure out how to write a function which would allow the user to zoom in on the screen. My program uses Windows BGI. If you follow this link (http://csci.biola.edu/csci105/using_winbgi.html) you'll find winbgi2.cpp and graphics2.h, both of which are in my program, and I've also attached my ShapeGeometry.cpp and ShapeGeometry.h in case they are of any use to anyone. Any help would be much appraciated!