Explanation:
always think what are 100% and 1%, and then find the desired % by comparing with either the 100% or 1%.
in our case here
100% = $3000.
1% = 100%/100 = 3000/100 = $30.
$213 are how many % of $3000 (100%) ?
as many % as times that 1% fits into 213 (that is what division is for) :
213 / 1% = 213 / 30 = 7.1%
sales tax rate = 7.1%