The rate over $9000 is 6%. Unadjusted gross $64,000 x .06 = $3840.
The 6% is applied to net income, after applying deductions to Missouri adjusted gross income (AGI). Using as an example a married couple with two dependents, if AGI is $64,000, Fed tax $7,500, and standard deduction, net income is $39,000.
AGI $64,000
deduction exemption $4,200, Fed tax $7,500, standard deduction $10,900, dependents $2,400, total $25,000.
Net income $64,000 - $25,000 =$39,000
The Missouri tax is computed separately for each spouse, with $315 paid on the first $9,000. So if the $39,000 was split $20,000 for one spouse and $19,000 for the other, the tax would be $1,890.
Spouse 1: $20,000 - $9,000 = $11,000. $11,000 * 6% = $660. $315 + $660 = $975.
Spouse 2: $19,000 - $9,000 = $10,000. $10,000 * 6% = $600. $315 + $600 = $915.
$975 + $915 = $1,890.
Where are you getting your information?
You are not even close to actual known fact.