According to zillow.com, the property taxes of my grandparent's house is $4,778 as of June,2011. The tax was $4,524 when my family moved in during October 2008.
To find the model, you go to STAT ---> EDIT---> Plug in X & Y values. The X values would be from 2004-2011 but I type in #1-8 represent years. The Y values would be the property taxes. Then I found Quart Reg fits the best after I tried all four Reg ( Linear, Quad, Cubic, Quart) because R^2 is closet to 1.
My house property taxes will not be $100,000 because the maximum in $4,800.
The prediction can be changed if the value of the house increases.