$489 - $405 = $84
$900 - $500 = $400
Thus, from every dollar of merchandise she sells, she gets $(84 / 400)
$1700 - $900 = $800
Thus, she will earn $(84 / 400) * (800) = $168 more.
Hence, her salary for selling $1700 worth of merchandise is:
$489 + $168 = $657