Lesson objective: Determine the equation of a line given two points, including vertical and horizontal lines.

This lesson helps to build procedural skill with slope calculation and solving for the *y*-intercept. Algebraic equations are used here to help students focus on the procedure of calculating slope and solving for *y*-intercept when it is not easy to reason through these components using a graph, table, or verbal description. This work develops students' understanding that the slope is equivalent between all pairs of points on a given line, so only two points are needed to define a linear function.

Students engage in Mathematical Practice 6 (Attend to precision) as they calculate slope and *y*-intercept, being careful to adhere strictly to the definition of slope, and not mix up the *x* and *y* values, or the order of their values when calculating differences.

**Key vocabulary:**

- horizontal line
- slope
- undefined slope
- vertical line
*y*-intercept