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