how to print screen on mac keyboard?

i have a pc and i use a mac keyboard.

command+shift+4

Don't do that. Use a PC keyboard. They cost about $10. BTW, you should make it clear what you are using, as in I am using Windows 7 or I am using OS X on a hackintosh. For all the six different Mac key combos for screen shot (non-Windows users say screen shot instead of PRNT SCRN), see my Best Answer link below.