In a two dimensional motion of a particle, the particle moves from point A, position vector $\sqrt{15}$ to point B, position vector $r_2.$ If the magnitudes of these vectors are respectively,  $r_1=3and{r}_2=4$ and the angles they make with the x-axis are ${\theta }_1={75}^{0}and{15}^0,$ respectively, then find the magnitude of the displacement vector