Let x be the time in hours and y be the distance driven in miles. If x = 0, then no time has passed which means he can't drive anywhere. So y = 0 when x = 0. The first point is (0,0). Note: the y intercept is 0 as this is where the graph crosses the vertical y axis.
One hour later, he has driven 35 miles because 140/4 = 35. In other words, his speed is 35 miles per hour (mph). So we have the second point (x,y) = (1,35). Note: the rate of change is the same as the slope in this problem. Also the rate of change is the same as saying "speed" for this problem.
Plot the two points (0,0) and (1,35) and draw a straight line through the two points. Extend the line as far as you can. The graph is linear assuming he maintains his speed to be the the same throughout the whole trip.
