Let $\vec v$ and $\vec w$ be vectors in $\mathbb{R}^6$ such that:

$$\lVert v\rVert=2$$

$$\lVert w\rVert=5$$

$$\lVert v+w\rVert=4$$

In the triangle whose vertices are the origin and the endpoints of $\vec v$ and $\vec w$, what is the length of the side opposite the origin?