If the angle between the lines $\dfrac{x}{2}=\dfrac{y}{2}=\dfrac{z}{1}$ and $\dfrac{5-x}{-2}=\dfrac{7y-14}{p}=\dfrac{z-3}{4}$ is $\cos^{-1}\left(\dfrac{2}{3}\right)$, then $p$ is equal to :