A birds nest is 12 feet above the ground. A moles den is 12 inches below the ground. What is the difference in height of these 2 positions?

That would be 24 ft. Imagine the origin which is 0 and the birds nest is + 12 since its above the ground and the moles den is - 12 since its under the ground. So it would be + 12 - (-12) which you can write as 12+12 which equals 24.