Two particles having position vectors \[\overrightarrow{{{r}_{1}}}=(3\hat{i}+5\hat{j})m\]and\[\overrightarrow{{{r}_{2}}}=(-5\hat{i}-3\hat{j})m\]are moving with velocities\[\overrightarrow{{{v}_{1}}}=(4\hat{i}+3\hat{j})m{{s}^{-1}}\]and \[\overrightarrow{{{v}_{2}}}=(a\hat{i}+7\hat{j})m{{s}^{-1}}\]. If they collide after 2 s, the value of a is