Answer:
Darren increasse the volume of his toolbox by a factor of 8, not 2 (double).
Explanation:
The volume of a box is (length)*(width)*(height) or l*w*h = Volume
Original volume: (9)(4)(7) = 252 in^3
New volume: (18)(8)(14) = 2016 in^3
Darren would increase the volume of his toolbox by (2016/252) or a factor of 8. Technically, Darren did double the volume of his toolbox. But I suspect he'll be surprised that the increase was a factor of 8, not 2.