By looking at the graph we can see that f(x) increase by 1 (on the vertical axis . or y-axis ) from 2 to 3, over the interval 0 to 1 ( horizontal axis, or x-axis)
It increases by 2 over the interval 1 to 2.
by 4 over the interval 2 to 3.
And by 8 over the interval 3 to 4.
Therefore this exponential function grows by a factor of 2 (since it doubles every by every interval of 1)