First using Equation 12: we can solve for Cp0.
If 0.5 = Cp0 x e-0.21 x 6 then Cp0 = 1.8 mg/L.
Since Cp0 = Dose/V we can solve for Dose = Cp0 x V = 1.8 x 230 = 405 mg approximately 400 mg which give a concentration at 12 hours of 0.49 mg/L.
HINT: Notice the 'correct' answer for dose according to the calculation is 405 mg, then using 'professional' judgement we might round this answer to 900 mg. Thus, there are two answers here. The calculated answer and the rounded answer.
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And the answer graphically