The graph below plots a function f(x):

if x represents time, the average rate of change of the function f(x) in the first two seconds is

So, delta x is the same everywhere, it's a linear equation.

The formula is delta f(x) / delta x.

Theorically is the same as of the slope formula.

You can do 50/2 = 25 <- answer

But if you prefer with the slope formula, where f(x) = y

You see the rise is at (0, 50) and the run at (2, 100)

(y2-y1)/(x2-x1) = (100-50)/(2-0) = 50/2 = 25