Both Tom and Jerry can mow the estate in 3 hours
Jerry alone can mow the estate in x hours; Jerry's rate = 1/x mow/hour
Tom would mow the estate in 2x hours; Tom's rate = 1/2x mow/hour

Jerry would take 4.5 hours to mow the estate, while
Tom would take 2x4.5 = 9 hours to mow the estate.