Question: what is the slope of the line joining?
Answer: Let (x1, y1) and (x2, y2) be two given points, and let (x, y) be any point on the line. Then the slope joining one of the given points and any point, is equal to the slope joining the two given points, because a line has one and only one slope. This is called the two-point formula for the equation of a straight line.