In this problem, you are asked to write a number in scientific notation. In figures, 78 million would look like this:
78,000,000
To write this to scientific notation, you have to move the decimal point to the left until you will get a number between 1 and 10.
Here, you will have to move the decimal point 7 times to get 7.8
The exponent of the scientific notation is the number of times you moved your decimal point. Since the movement of decimal point is going to the left, you will have a positive exponent.
The scientific notation would look like this:
7.8 x 10^7