[tex]\displaystyle\bf\\d=4\\\\S_{12} =336\\\\\\S_{12} =\frac{a_{1} + a_{12} }{2} \cdot 12=(a_{1} + a_{12} )\cdot 6=(a_{1} + a_{1}+11d )\cdot 6=\\\\\\=(2a_{1} + 11d )\cdot 6\\\\336=(2a_{1} +11\cdot 4)\cdot 6\\\\56=2a_{1} +44\\\\2a_{1}=12\\\\\boxed{a_{1} =6}\\\\a_{9} =a_{1} +8d=6+8\cdot 4=6+32=38\\\\\boxed{a_{9}=38}[/tex]