How do you write 646,000 in scientific notation?

$6. 46 \times {10}^{5}$
Move the decimal five places to the left to leave only one digit to the left of the decimal. Keep the three significant digit 6,4,6 . The zeros are simply place holders and are not significant. Then multiply by ${10}^{5}$ to keep the numerical value of the number the same
$646 , 000 = 6.46 \times {10}^{5}$