What is the square root of -28?

Go 7 units of distance in any direction. Turn 90 degrees clockwise and go 7 units of distance in a straight line. Turn 90 degrees clockwise and go 7 units of distance in a straight line. Turn 90 degrees clockwise and go 7 units of distance in a straight line.

You will have travelled along a square route whose total distance is 28 units. Alternatively you could have turned counter clockwise each time.