A particle is moving in $xy$ plane. Its velocity and acceleration at a certain instant is given by $\overline{V}=3\widehat{i}+4\widehat{j}m/s$ and $\overline{a}=5\widehat{i}+\frac{15}{4}\widehat{j}$ .Find the rate of change of speed (in ms^-2) of particle at that instant___