To find the common ratio, divide the 2nd and 1st terms to get 20. This is the ratio. Multiply this by 800,000 to get 16,000,000, and do this again to get 320,000,000. These are the next two terms.
When adding or subtracting in scientific notation, all that is required is that the exponents of all the numbers be the same. The hint says make all the exponents be 0. Above, we have chosen to make them all be 4. You could also use 3:
