$26.21 x 0.0825 = $2.16 is the sales tax paid if not included in the $26.21 (Total paid would be $26.21 + 2.16 = $28.37)
If $26.21 includes the sales tax:
x + 0.0825x = $26.21
1.0825x = $26.21
x = $26.21 x = $24.21 To check: $24.21 x 1.0825 = $26.21
1.0825 Amount of Sales tax paid: $26.21 - $24.21 = $2.00