In a two dimensional motion of a particle, the particle moves from point $A$, position vector $\vec{r}_{1}$, to point $B$, position vector $\vec{r}_{2}$. If the magnitudes of these vectors are, respectively, $r_{1}=3$ and $r_{2}=4$ and the angles they make with the $x$ -axis are $\theta_{1}=75^{\circ}$ and $\theta_{2}=15^{\circ}$, respectively, then find the magnitude of the displacement vector.