Need Help with Econ!!!!!!!?

Hi. I'm currently taking econ-macroeconomics in college and I have a midterm tomorrow and I need to get some things straight. So we're gonna writing essays about an economy that the professor makes up. For example using the y=[a+b(y-tax)]+I+G formula where you gotta find Y aka GDP. and Yf is given. Yf is the preferred GDP.In order to figure out budget surplus, deficit, balanced; do I compare Y with Yf or tax with G. like for example the equation Y=500+.9(y-600)+800+800 and Yf 15000. Y ends up being 15600. So would I compare 15000 and 15600 or 600 and 800? Cuz if I compare 15000 and 15600 there would be a surplus and if I compare 800 and 600 there would be a deficit. Help?