Answer
$39.7mg \space C_{21}H_{26}N_{2}S_{2}$ for one tablet
Work Step by Step
Let's use the entire 12 tablet data and divide by the answer by 12 to get the initial tablet thioridazine content
We are given $.301g$ BaSO$_{4}$ so we need to work backwards in the way the reaction occurred because we started with Thioridazine and ended with BaSO$_{4}$
$\frac{0.301g\space BaSO_{4}}{1}\times\frac{1 mol\space BaSO_{4}}{233.4g\space BaSO_{4}}\times\frac{1 mol\space SO_{4}^{-2}}{1 mol\space BaSO_{4}}\times\frac{1 mol\space C_{21}H_{26}N_{2}S_{2}}{1 mol\space SO_{4}^{-2}}\times\frac{370.2g\space C_{21}H_{26}N_{2}S_{2}}{1 mol\space C_{21}H_{26}N_{2}S_{2}}\times\frac{1000mg}{1g} = 477mg\space C_{21}H_{26}N_{2}S_{2} $
This mass is for 12 tablets so divide by 12 to get the mass for one tablet:
$477mg\space C_{21}H_{26}N_{2}S_{2} \div 12 = 39.7mg \space C_{21}H_{26}N_{2}S_{2}$
