Answer:
volume = length × width × depth
Assuming that the driveway has a rectangular shape, you can measure the length and width of the area to be covered and multiply by the depth of 3/4 ft (which is 0.75 ft) to get the volume in cubic feet.
For example, if the driveway is 20 ft long and 10 ft wide, the volume of concrete needed would be:
volume = length × width × depth
= 20 ft × 10 ft × 0.75 ft
= 150 cubic feet