I don't understand where they got this equation from I've used it in algebra but I don't understand how it applies to limits sorry it's sideways please help me out

Let f(I) = H Then the expression f (Ith) - f(z) can be written in the form I(x + h)' where A is a constant and A Using your answer from above we have: f'(z) = lim f(z th)- f(I) h-10 Finally, find each of the following: f'(1) = O f' (2) = f'(3) = Note: You can earn partial credit on this