A parallelogram is constructed on and where and and and are anti ...........

A parallelogram is constructed on $3\vec{a}+\vec{b}$ and $\vec{a}-4\vec{b}$ , where $|\vec{a}|=6$ and $|\vec{b}|=8$ and $\vec{a}$ and $\vec{b}$ are anti-parallel, then the length of the longer diagonals is