Question

A body of mass m at rest is subjected to a constant force f for time t. The kinetic energy at time t is given by

Answer 1

K.E=1/2*m*v^2
...........................
You can get the velocity using v=u+at...equations.
Hope you got.

Answer 2

Kinetic energy = 1/2mv^2
Now, by using v=u+at,Substitute it in place of V.Now,  You can find the solution:)