Answer
$t=1.22\times10^{-10}s$
Work Step by Step
From the first equation of motion,
$v_f=v_i+at$
$\implies t=\frac{v_f-v_i}{a}$.............eq(1)
Given that,
$v_f=\frac{c}{10}=\frac{3\times10^8}{10}=3\times10^7\frac{m}{s}$
As initially electron is at rest so $v_i=0\frac{m}{s}$
Putting these values in eq(1) and solving:
$t=\frac{3\times10^7-0}{2.46\times10^{17}}$
$t=1.22\times10^{-10}s$
