I got a solution using DSolve as :
Function[{t}, -((
1. (2. + k (-1. - 2. t) + k^2 (-0.5 + 1. t + 1. t^2)))/k^3) +
E^(-1. k t) C[1]]
What does '.' mean and '()'? For example, in the case of '2.+k(-1.-2.t)', what does it mean? How to translate it to normal Math notation?