I'm assuming you're talking about the limit definition of a derivative.
This is a weird problem because I don't know how in depth or rigorous your teacher wants of an answer. I would point out the derivation of the terms in the formula.
Assume that P(a,f(a)) exists and that f(x) is a function that is continuous in the neighborhood around x=a. If we want to approximate the slope of the tangent line at x=a, then we can use a second point very close to "a" an estimate. This is where "a+h" comes from. Then you should fill in how the formula uses the simple formula for the slope of a line and where the terms come from. Now I would say that as we take "a+h" to be close to "a", the approximation becomes better and better. This is where the limit part comes in.
I hope that helps some. I don't know how much you already knew but I can do another way if you need.