#### Answer

$1.43\times10^{-4}s$

#### Work Step by Step

We use the following equation of kinetic motion:
$v^{2}=u^{2}+2aS$
$350^{2}=210^{2}+0.08a$
$a=\frac{560\times140}{0.08}=9.8\times10^{5} m/s^{2}$
Now, $v=u+at$
$t=\frac{v-u}{a}=\frac{140}{9.8\times10^{5}}s=1.43\times10^{-4}s$