4503599627400000 

Put a decimal between the first digit 4 and the 5 next to it:

 4.503599627400000

Now count how many digits there are after the decimal.
I count 15.  Remember that number, 15.

 Drop off all the trailing zeros at the end:

 4.5035996274

Remembering that number 15, put on ×1015  

 4.5035996274×1015      <-- final answer

Edwin