Without algebra:

The LCM of 8 hours and 12 hours is 24 hours.

So in 24 hours, the first crew can seal 3 parking lots.
And in 24 hours, the second crew can seal 2 parking lots.

So together they can seal 5 parking lots in 24 hours.
So together they can seal 1 parking lot in 24/5 = 4.8 hours.

With algebra.

Suppose it takes them x hours working together.

The first crew's rate is 1 parking lot per 8 hours or 1/8 parking lot per hour.

The second crew's rate is 1 parking lot per 12 hours or 1/12 parking lot per hour.

Then in x hours the first crew does (1/8)x of the sealing and the second
crew does (1/12)x of the sealing.

Then their total is 1 driveway sealed:

(1/8)x + (1/12)x = 1 

     (1/8+1/12)x = 1

    (3/24+2/24)x = 1

         (5/24)x = 1

              5x = 24

               x = 24/5 = 4.8 hours.

Edwin