A student investigated how the terminal velocity of steel spheres falling through a viscous liquid varies with their diameter.
The student plotted the results on a graph, shown below, but did not finish drawing the line for diameters larger than 6 mm6\text{ mm}6 mm.

Estimate the terminal velocity for a steel sphere with a diameter of 9 mm9\text{ mm}9 mm.
Show how you obtain your answer.