4.5 billion minus 146 million equals 4354000000. Then you must express it in scientific notation. To do this you move the decimal point so that there is only one number in front of it. Then count how many spaces you moved it and that will be your exponent. If you moved the exponent to the left then the exponent is positive, if you moved the point to the right then the exponent is negative. The format in scientific notation is: all of the numbers are included until the rest are zeros x 10^(to the power of) the number of places you moved the decimal.
So, in this problem, your answer is A.
