Step-by-step Solution:

\(\vec{a}+\vec{b}=-\vec{c}\) Squaring both the sides \[\] \(|\vec{a}|^{2}+|\vec{b}|^{2}+2 \vec{a} \cdot \vec{b}=|\vec{c}|^{2}\) \[\] \(9+25+2 \cdot|\vec{a}| \cdot|\vec{b}| \cdot \cos \theta=|\vec{c}|^{2}\) \[\] \(34+2.3 \cdot 5 \cdot \cos \theta=49\) \[\] 30. \(\cos \theta=15 \Rightarrow \cos \theta=\frac{15}{30}=\frac{1}{2}=\cos 60^{\circ}\) \[\] \(\Rightarrow \theta=60^{\circ}\) \[\] Choice (b)\[\]