Vector Algebra

Let $(\mathbf{a}, \mathbf{b})$ denote the angle between vectors $\mathbf{a}$ and $\mathbf{b}$. If $\mathbf{a}=2 \hat{\mathbf{i}}+3 \hat{\mathbf{j}}+6 \hat{\mathbf{k}}, \mathbf{a} \cdot \mathbf{b}=4$ and $(\mathbf{a}, \mathbf{b})=\cos ^{-1}\left(\frac{4}{21}\right)$, then $\mathbf{a}+\mathbf{b}=$