Starting with the numerical calculations in Example/Checkpoint 2.1 (Sec 2.1), we reviewed slopes of secant lines and the definition of the derivative, and used it to calculate f'(1) for f(x) = x^2:

The definition of the derivative is covered in Sec 3.1 of the textbook. Please read the start of that section, including Figures 3.3 and 3.4, which is another presentation of the figures you were asked to draw for HW#1: