Crew 1’s rate is 1/9 parking lot per hour and Crew 2’s rate is 1/12 parking lot per hour.

Together their combined rates are 1/9+1/12 parking lots per hour=4/36+3/36=7/36.

That means they can seal 7 parking lots in 36 hours, so one parking lot can be sealed in 36/7=5.14 hours approx.

Another way is to imagine the parking lot subdivided into 36 sections. If Crew 1 takes 9 hours to seal 36 sections then in 1 hour it can seal 4 sections; and in 1 hour Crew 2 can seal 3 sections. Together they would be able to seal 7 sections between them in 1 hour. In 5 hours they would seal 35 sections. That leaves one section to seal, and that would take 1/7hr, because they can seal 7 sections in an hour.