Hint Consider the function $g(x)=f(x)-f(x-1)$, and the fact that it is continuous. What values does it take at $x=1$ and $x=2$ ? Consider applying Intermediate Value theorem of continuous functions.
Hint Consider the function $g(x)=f(x)-f(x-1)$, and the fact that it is continuous. What values does it take at $x=1$ and $x=2$ ? Consider applying Intermediate Value theorem of continuous functions.