The reason is, you haven't got the final answer yet, as your formula still has an integrating constant C.
Note that exponential function has a following feature
e^a e^b = e^(a+b).
Find the value of C in your function remembering that for
t = 0
velocity was also zero
V(0) = 0.
The relevant equations should be
Ft = ma = W - kV
W = mg
a = dV/dt
m dV/dt = mg - kV (1)
This differential equation has an easy to guess special solution
V = mg/k = const.
The homogeneous equation for the one above is
m dV/dt = -kV (2)
Note that it is similar...