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Let $f$ be a linear function for which $f(5)-f(2)=0$. What is $f(8)-f(2)$? (  Adapted From 2003 AMC 12B Problems, Question \#9) 
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$f(5)-f(2)=0 \Rightarrow f(5)= f(2)$; thus, $f(x)$ is a constant function. Then, $f(8) - f(2) = 0$. 