In this video, the instructor shows how to find an unknown coordinate given the other coordinate of that point and the equation that passes thought the point. A point is said to lie on the graph, if the point can satisfy that equation of the line. To check if a particular point satisfies an equation, all you have to do is substitute the value of the point in that equation and check if it validates the equation. Now, substitute the given point with an unknown coordinate in that equation. If the point lies on that line, then the equation should evaluate to true. So, simplify that equation giving out the value of the missing coordinate. This video shows how to find a missing coordinate in a point that lies on a given equation.