Answer
$1.50\ g\ C$
Work Step by Step
1. Using the first sample, subtract the remaining elements from the mass of HCN in order to determine the mass of carbon
2. \begin{align}
m_{HCN}=m_{H}+m_{C}+m_{N}\\
m_{C}=m_{HCN}-m_{H}-m_{N}\\
(7.83-0.29-4.06)g= 3.48\ g
\end{align}
3. Then use the ratio $m_{C}\ :m_{HCN}$ from the first sample as a conversion factor in order to calculate the mass of carbon from the second sample.
4. $3.37\ g\ HCN\times \frac{3.48\ g\ C}{7.83\ g\ HCN}=1.50\ g\ C$
