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Let $f$ be a linear function for which $f(6)-f(2)=12$. What is $f(12)-f(2)$? (  2003 AMC 12B Problems, Question \#9) 
Overseas Competition->Knowledge Point->Calculation Modules
Let $f$ be a linear function with slope $m$.  $$ \begin{gathered} m=\frac{f(6)-f(2)}{\Delta x}=\frac{12}{6-2}=3 \textbackslash\textbackslash{} f(12)-f(2)=m \Delta x=3(12-2)=30 \Rightarrow(D) \end{gathered}$$ 