Section 1.1 and 1.3-Work with difference quotient
Work with difference quotient, except using y rather than h which is how the difference quotient is typically presented.
See
https://www.khanacademy.org/math/calculus/differential-calculus/secant-line-slope-tangent/v/slope-of-a-line-secant-to-a-curve
for videos about why we will need this topic.