\[a_{1} = 3;\ \ a_{2} = 7:\]
\[d = a_{2} - a_{1} = 7 - 3 = 4.\]
\[a_{12} = a_{1} + 11d = 3 + 11 \cdot 4 = 47.\]
\[S_{12} = \frac{(3 + 47) \cdot 12}{2} = 50 \cdot 6 = 300.\]
\[Ответ:a_{12} = 47;\ \ S_{12} = 300.\]