To find the equation that represent the land of the plane (the blue line) we have to find the slope and the interseccion with the y axis.
So in the graph we can see that the intersection is:
Now to find the slope que can use the green points that made a parallel line to the blue line, so with the coordinates (8,-2) and (10,-1) we can use this equation:
and we replace the coordinates so:
So the final equation will be: