d=rt is d=2r when t=2 hours. This can be graphed as a straight line passing through the origin and a slope of 2. To draw the line take any value of r, say, 4 and work out the d value=8. That gives us the point (4,8) using r as the horizontal axis and d the vertical axis. Now draw the line through (4,8) and the origin and that gives the graph. When d=2r, r=½d shows how d is expressed in terms of r and r is expressed in terms of d.