a) $cos(\overrightarrow{a},\overrightarrow{b})=\frac{-3.2+1.6}{\sqrt{3^{2}+1^{2}.\sqrt{2^{2}+6^{2}}}} = 0$
=> $(\overrightarrow{a},\overrightarrow{b})=90^{o}$.
b) $cos(\overrightarrow{a},\overrightarrow{b})=\frac{3.2+1.4}{\sqrt{3^{2}+1^{2}.\sqrt{2^{2}+4^{2}}}} = \frac{1}{\sqrt{2}}$
=> $(\overrightarrow{a},\overrightarrow{b})=45^{o}$.
c) $cos(\overrightarrow{a},\overrightarrow{b})=\frac{-\sqrt{2}.2+1.(-\sqrt{2})}{\sqrt{2+1^{2}.\sqrt{2^{2}+2}}} =-1$
=> $(\overrightarrow{a},\overrightarrow{b})=180^{o}$.