what is the approximate square mileage of the earth's total surface (land and water)?

Answer 1

Surface area: 510,065,600 km²
Land area: 148,939,100 km² (29.2 %)
Water area: 361,126,400 km² (70.8 %)

These are square kilometers, you can do the conversion. If you don't know how to do that, then you will never be a scientist, and you should go back to watching Dancing with the Stars.

Answer 2

The area of a sphere is 4 pi r^2.
If we make the simplifying assumption that the Earth is a sphere with a diameter of 7926 miles (radius = 3963) then the area would be 4*pi*3963^2 = 197359487 mi^2