This Instructional Activity is a part of the Lesson 15, Unit 4, Grade 8. Suppose a non-vertical line is given in the coordinate plane. As before, we mark a point P on the line and go one unit to the right of P and mark point Q. Then, we draw a line, through point Q that is parallel to the y-axis. We mark the intersection of lines. Again, recall that we consider the line to be a vertical number line where point Q is at zero.