Angle between vectors to find a component

If the angle between the vectors $2\tvec{i} - \tvec{j} + 2\tvec{k}$ and $2\tvec{i} + m\tvec{j} + 6\tvec{k}$ is $\cos^{-1}\left(\frac{13}{21}\right)$, then the value of $m$, where $m \in R^+$, is