With the alternative form of the derivative as an aid, make sense of the following limit expression by identifying the function f and the number a. So right here, they want to find the slope of the tangent line at 5. Here, they wanted to find the slope of the tangent line at a. So it's pretty clear that a is equal to 5 and that f of a is equal to 125. Now, what about f of x? Well, here, it's the limit of f of x minus f of a. Well, here's the limit as x to the third minus 125.