How to write 645300 in expanded form using exponents

The number 645,300 is is already in the expanded form. I think what you mean is in specific notation, because in this form, we make use of exponents. To convert this, simply move the decimal place to the left until before you get to the last digit. The number of places moved is equivalent to the exponents, and the + sign is for movement to the right, and the - sign is for the movement to the left. Thus, the answer would be 6.453×10⁻⁵.