US postal service delivered = 7.14 x 10^10 pieces of mail in December.
US postal service delivered = 3.21 x 10^10 pieces of mail in January.
We need to find the total number of mail delivered in both months.
In the shown work you setup problem correctly.
You are adding
7.14 x 10^10
3.21 x 10^10
_____________
10.35 x10^10
You got 10.35 x10^10 on adding.
Therefore, second step is correct too.
But in third step when you moved decimal one place on left you wrote 1.035 x 10^10.
Error: When we move decimal one place on left the exponent of 10 should be raised to one.
Correct work: After moving decimal one place on we would get left 1.035 x 10^11.
Strategy used: Moving decimal one place left raise the power 10 to one and moving decimal place right decrease the power 10.
