A particle of mass m amd charge q is theown at a speed u against a uniform electric field E . how much distance will it travel before coming to momentary rest ?

Dear Student,
Retarding force acting on the particle, $F=-qE$
The particle will decelerate with a value, $a=-\frac{qE}{m}$
initial speed, u
final speed, v =0.
Let the distance travelled by the particle before comming to momentary rest be s.
Applying the following equation for straight line motion:
$$\begin{gathered} v^2=u^2+2as \\ {u}^2=-2as=-2\times \left(-\frac{qE}{m}\right)s \\ s = \frac{u^2}{2\times \left(\frac{qE}{m}\right)}=\frac{m{u}^2}{2qE}. \end{gathered}$$
Regards