check the picture below, you can pretty much count the units off the grid.
recall that A = (1/2)bh.
It's 3x+2 because 3 ÷2 is 6 so it's 3x+2
If it moves 423 feet in 7 minutes, you divide 423 by 7 to see how much it moves per minute, which is Around 60.4 feet per minute