​Can someone explain this to me?

The \(\Delta \)symbol means take the difference between successive values of ...

So \(\Delta f(x) \)means take the difference between successive values of f(x): namely 3 here, and \(\Delta x\)means take the difference between successive values of x, or 1 here.

So \(\frac{\Delta f(x)}{\Delta x}=\frac{3}{1}\rightarrow3\)